Back taxes
Prior years that were never filed, what the penalties and interest actually come to, and the order to file them in so you can get current.
Paying what you owe: installment agreements and the options
If you cannot pay the IRS in full, file anyway, then choose between a short term plan, an installment agreement, and in rare cases an offer in compromise.
What an IRS notice means, and which ones need an answer
Most IRS notices are proposals, not verdicts. Read one before you pay it, answer a CP2000 on its own response form, and never let the reply date pass.
Penalty abatement: first time abatement and reasonable cause
The IRS applies first time abate before it looks at reasonable cause, which can spend a one time waiver on a year that did not need it. Order matters.