My children's father & I had lived & been together for 10 years. He died before we were able to legally marry. Can I claim widow? Is there any benefits to doing so?

To qualify as married, you must have been legally married per the laws of the state where the marriage took place. The IRS recognizes marriages as valid if they are legal in the state where the marriage occurred. Based on what you mentioned, you would not be eligible to file a joint return with your late significant other unless you live in one of the few states that recognize common law marriage and you meet the criteria for the state to recognize a valid marriage. Since you don't reference the possibility of a common law marriage, it seems unlikely that you would be considered married for tax purposes. Also, if you were considered legally married in 2019, you must file as married for 2019, either jointly (as a surviving spouse) to include both your incomes, or married filing separately.

You would only use Qualifying Widow for the two years after the year in which your spouse passed (e.g. if he passed in 2019, you would be eligible to claim Qualifying Widow for 2020 and 2021). You must also be able to claim a child or stepchild as a dependent to claim this filing status, though it seems implied that you would by your question. There are other requirements, but those are best reviewed on the IRS website. However, filing as a widow without having filed a married return is likely to raise an eyebrow at the IRS.

There isn't much information provided in your question to give a comprehensive answer, but the IRS website (irs.gov) does have a lot of information on this topic. The best places to look for the requirements for filing status are in the instructions for the Form 1040, Publication 17 (essentially the big instruction book for the Form 1040), or Publication 501 which focuses on dependent, standard deduction, and filing information. All of these sources provide definitions and requirements for each filing status.

Hope this helps.