If you are talking about 2012 tax liabillity, all you have to do is to file as "married filing singly" so his tax & penalties from his pre-mature withdrawal from his 401k will not be your responsibility. Any tax returns you file jointly, the tax owed are joint & severally owed by you and him. Talk to your divorce attorney about the other issues. Good luck.
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You are jointly and severally liable for any income tax liability associated with a joint return that you signed unless one of the very limited innoncent spouse exceptions applies, which does not appear to be the case here. To limit liability for 2012, file seperately.
You have several different issues here. They may be answered through the divorce court or through the use of a tax professional.
If you file your returns married filing separately for tax year 2012, you shouldn't have to pay the taxes on his 401k.
In your divorce decree, you could have him pay the taxes or reimburse you if you pay.
Regarding the issue of claiming your child as dependent, typically that issue is also spelled out in your divorce decree. Some people alternate which spouse claims the exemptions. Some specify that one spouse takes the exemption. In any event, whoever is taking the exemption, earned income credit, etc, should have the other spouse execute a waiver so that both spouses won't take the same exemptions. This will be a tax nightmare in a few years if you both take the exemption.
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