delivers the majority opinion of the Court affirming trial court on Issues 1 and 2.
[¶2] Chief Justice Robert A. MILLER delivers the majority opinion of the Court with respect to Issue 3.
[¶ 3] SABERS, Justice, writing the majority opinion with respect to Issues 1 and 2.
[¶ 4] Father appeals from an order which awards mother child support arrearages and orders father to reimburse mother for income tax dependency exemptions and medical expenses.
FACTS
[¶ 5] Richard and Jewell were divorced October 21, 1983. At the time they had three minor children: Angela, Justin and Rebecca. They reached an agreement which provided Jewell would have custody of their three minor children, subject to Richard’s right to visitation. The attorneys on appeal did not represent the parties when the agreement was formed and Richard’s attorney did not represent him in the trial court.
[¶ 6] The agreement provided Richard would pay $300 per month (later increasing to $400 per month) child support until the children reached the age of majority. It provided Richard would be entitled to declare two of the children as his dependents for income tax purposes, “assuming that he has remained current on child support.” It also provided:
[Richard] ... agrees to be responsible for the medical expenses of the minor children until such time as they reach ... majority, if [Jewell] through her employment is able to obtain medical insurance coverage on the children, [Richard] agrees to pay the deductibles as it pertains to the children. [Richard] does intend to acquire medical insurance and coverage for the children as soon as he can afford the same and will apprise [Jewell] when he has such coverage.
[¶ 7] In May 1986, the agreement was modified to set Richard’s child support obligation at $150 per month. The trial court also entered a judgment against Richard for $1,555 in past child support.
[¶ 8] The children lived with Richard much of the time between 1988 and 1993.1 Angela turned eighteen in 1992. While the children lived with him, Richard did not pay child support in full and did not move to modify child support or change the custody order until 1993.
[¶ 9] Richard moved for temporary custody of Justin and Rebecca on May 3,1993.2 Jewell moved for an order to show cause asserting Richard owed child support, had improperly claimed income tax exemptions for their children, and had not reimbursed her for the *806children’s medical expenses. She also sought 1993 attorney fees and her travel expenses to the hearing.
[¶ 10] The trial court found Richard owed $10,550 in child support and refused to retroactively modify his obligation.
[¶ 11] Richard claimed federal income tax dependency exemptions for two of the children from 1989 until 1992. The trial court refused to consider Richard’s claim that he had physical custody of the children and ordered him to reimburse Jewell $2,263 for the dependency exemptions because he was not current on his child support obligation when he took them.
[¶ 12] Richard obtained medical insurance for the children in 1988 but did not notify Jewell, so both parties carried insurance. The trial court ordered Richard to reimburse Jewell for $16,323.49 she paid in insurance premiums and “all other care.”
[¶ 13] Richard appeals.
[¶ 14] 1. Whether Richard must reimburse Jewell for the entire cost of health insurance or only deductible amounts.
[¶ 15] The parties’ agreement indicated Richard would pay medical expenses of the children. It also indicated that if Jewell obtained medical coverage through her employer, Richard would pay the deductibles for the children.
In determining the proper interpretation of an agreement incorporated into a divorce decree, a court must seek to ascertain and give effect to the intention of the parties. In determining the intention of the parties, a court must look to the language that the parties used. A court can also consider the construction actually placed on the agreement by the parties as evidenced by their subsequent behavior.
Kier v. Kier, 454 N.W.2d 544, 547 (S.D.1990) (citations omitted).
[¶ 16] Richard claims the decision that he pay the full cost of medical care for the children conflicts with the terms of the agreement. The trial court found the agreement placed “an affirmative duty,” which Richard had not met, to contact Jewell when he obtained coverage for the children. Richard admitted he did not notify Jewell and that both parties carried insurance for a number of years due to this failure to communicate. He claims Jewell had an affirmative duty to inquire whether he had insurance before she purchased. This is contrary to the language of the agreement, which specifically states Richard “will apprise Jewell when he has ... coverage.”
[¶ 17] The intent of the agreement was to allow Richard to obtain insurance coverage for the children when he could afford it. This is evidenced by the fact that Jewell purchased insurance for the children until Richard contacted her. The trial court gave effect to the parties’ intent and its order is affirmed.
[¶ 18] 2. Whether Richard is entitled to credit on past-due child support for time the children resided with him.
[¶ 19] Since July 1, 1989, South Dakota law provided for an abatement of a portion of child support “if a child spends more than twenty-nine consecutive days with the noncustodial parent.” SDCL 25-7-6.14; Houser v. Houser, 535 N.W.2d 882, 885 (S.D.1995). “Abatement essentially gives credit on future payments for extended visitations which have already occurred.” Houser, 535 N.W.2d at 886.
[¶ 20] Richard had a duty to pay child support monthly. While the children lived with him, Richard did not pay child support in full. “Any unpaid support bec[omes] an unpaid judgment against [the payor spouse] as a matter of law, not subject to retroactive modification.” Id. at 885 (citing SDCL 25-7-7.3; SDCL 25-7-7.4; Kier, 454 N.W.2d at 546). SDCL 25-7-7.3 provides: “Any past due support payments are not subject to modification by a court or administrative entity of this state[J”
[¶ 21] Richard claims he is entitled to credit or abatement of his child support obligation for the months the children resid*807ed with him.3 The trial court held Richard’s support obligation became final when due and could not be modified. Richard claims he is entitled to abatement because his wife admitted he had actual custody of the children during the years 1988 to 1993 for substantially longer than twenty-nine consecutive days. In Houser, we held a trial court could not retroactively modify child support obligations by abatement even if visits of over twenty-nine days with the father were proved. Houser, 535 N.W.2d at 886. Richard should have moved the trial court for modification or abatement of his child support obligation or for a change in custody. He did not and we affirm the trial court’s decision on this issue.4
[¶ 22] 3. Whether Richard was entitled to federal income tax exemptions.
[¶ 23] The trial court held Richard was not entitled to dependency exemptions because he was not current on his child support when he took the exemptions. Richard claims he was entitled to dependency exemptions for the children because he claims he was the custodial parent. However, the agreement provided that Jewell was the custodial parent. “ ‘Custody’ ... [is] determined by the terms of the most recent decree of divorce or separate maintenance[.]” T.Reg. § 1.152-4.
[¶ 24] Noncustodial parents are allowed to take the exemption if a qualified pre-1985 agreement exists and the noncustodial parent provides at least $600 per year in support. IRC § 152(e)(4)(A). However, this agreement places a condition on Richard’s ability to claim the exemptions. Its terms are clear that Richard shall be entitled to declare two of the children as his dependents, “assuming that he has remained current on child support.” The trial court found he was not current.
[¶ 25] Generally, the purpose of IRC § 152(e) is to provide certainty to the parties taking the exemption, and no “implied exceptions” are made when a party has not been in compliance with the divorce decree. McClendon v. Comm’r, 74 TC 1, 1980 WL 4557 (1980). However, in Flatt v. Comm’r, 52 TCM (CCH) 713, 1986 WL 21704 (1986), the parties had agreed that if the father maintained hospitalization insurance for the children and a life insurance policy naming the children as beneficiaries, he could claim the two children as dependents for income tax purposes. The tax court held it could determine whether the father had been in compliance with the divorce decree because:
where a divorce agreement CONDITIONS the claim for dependency exemptions upon the performance of specific obligations, it is appropriate for [the tax court] to determine if, in fact, the party obligated to meet such conditions has fully complied.
Matt, 52 TCM (CCH) 713.
[¶26] In the present dispute, the trial court found the divorce court’s order “specifically provided that [Richard] was to pay child support before he could take any tax deductions.” The trial court also determined Richard was in arrears on his child support. Therefore, the trial court did not err in determining Richard was not entitled to claim the exemptions. We affirm.
[¶ 27] Both parties have filed motions for attorney fees in proper form, but we decline both motions.
[¶ 28] MILLER, C.J., and AMUNDSON, KONENKAMP, and GILBERTSON, JJ., concur as to Issue 1.[¶ 29] KONENKAMP and GILBERTSON, JJ., concur as to Issue 2.
[¶ 30] MILLER, C. J., and AMUNDSON, J., dissent as to Issue 2.
. Richard alleged, and Jewell did not contest that from 1988 to 1993, Angela lived with Richard for 40 months and Jewell for 2 months; Justin lived with Richard for 47 months and Jewell for 20 months; Rebecca lived with Richard for 12 months and Jewell for 54 months.
. In an order filed April 8, 1993, the trial court modified child support so that Richard's obligation was $354.84 per month for Justin and Jewell's obligation was $205 per month for Rebecca, resulting in monthly payments of $150 per month from Richard to Jewell.
The trial court also ordered that any medical costs not paid by insurance be paid 59% by Richard and 41% by Jewell.
. At the hearing, the parties appeared to agree that some type of offset would be available to Richard, but did not agree on the amount. Parties are without authority to modify or forgive child support arrearages without court approval. Houser, 535 N.W.2d at 885 (citing Vander Woude v. Vander Woude, 501 N.W.2d 361, 364 (S.D.1993)).
. Richard claims in his reply brief that South Dakota's automatic judgment statute violates his due process rights because it imposes a detriment without any opportunity for a pre-deprivation hearing. A party may not raise an issue for the first time on appeal, especially in a reply brief when the other party does not have the opportunity to answer. Finch v. Northwest School Dist. No. 52-3, 417 N.W.2d 875, 878 (S.D.1988).