Yaelle, this is good advice. If you have a CPA or tax professional it would be good to review your situation.
If you have not filed a year and need to, file. If the IRS filed a Substitute for Return on your behalf you need to respond with your return. The IRS is always going to assess your tax in the least advantageous way. After that, get a loan pay it off or set up an Installment agreement with the IRS.
That is simply not true.
The IRS may send you a document and REQUEST that you sign agreeing to a specified "imputed income" based on past returns. It is not a tax return in any way shape or form. The IRS just wants you to believe it is a tax return they filed for you.
That form is nothing more than a CONTRACT that you have no duty to sign or return, no matter what an IRS collection agent tells you. If you sign it, it is likely enforceable, but maybe not if you think it is coercive. Best not to sign it.
I was sent two of these documents many years ago. Ignored the first one. When I got the second one, I called the collection agent and we had a relatively short, and mostly pleasant conversation.
My first question was, "Is this a contract?" The agent agreed that is was, so I asked him if understood that the U.S. Constitution does does permit people to be forced to sign any contract. He reluctantly agreed.
I asked this IRS agent a few more questions and requested that he get back to me when he had the answers. He also admitted that he had a quota on the amount of money he was required to collect. I told him that I did not owe taxes at that time. Furthermore, I simply wasn't worth the effort it would take him to only to come up empty-handed. I haven't heard from the IRS in over 15 years because they have no good answers to a few very basic questions.
BTW, I pay every dime of taxes that I owe.