I have been filing single. I am married but we file separately and I didn't have his ssn number. Are there any advantages to filing married/ separately? Does it matter?

[mell113]

I don't want to raise any flags, we just have always kept things separate. It dawned on me that the IRS may want me to file as married so I just need guidance on any implications

[DanielV01]

Yes, it does matter.  If you are married you legally must file either married filing joint or married filing separately.  For most, married filing joint is the most advantageous status, as this FAQ shows:  https://ttlc.intuit.com/replies/3288477.  It is a choice, however, and there are couples who have good reason to file separately.  Legally, you cannot file single.  But to file a joint return you do need his ssn (which, of course, you'll also need on a joint return as you'll be filing and signing together.)