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@chenschuh wrote:

He found out it's cheaper for him to buy health ins through market.

When I went to form 8962 enter month of marriage for alternate marriage calculation smart worksheet enter 07, it totally change the outcome of tax return.

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Are you saying that he had Marketplace insurance AFTER he got married and was eligible for his wife's employer insurance?  If so, he generally does not qualify for any Premium Tax Credit after he became eligible for employer insurance.  You would change the SLCSP to $0.01 for each month that he was eligible for affordable employer insurance.

If the taxpayer needs to repay some of the Advance credit, the Alternative Calculation in the year of marriage can sometimes reduce that repayment.