When you are covered on the first day of December, then you can invoke the last-month rule. the last-month rule lets you use the full annual HSA contribution limit, regardless of how many months you had HDHP coverage.
So if you had Self-only coverage on December 1, 2022, then your annual HSA contribution limit for 2022 was $3,650, even though you were under the HDHP for just one month.
The catch is that you must stay under HDHP coverage for the next year (i.e., all of 2023).
As for the lines, I assume that you are referring to the 8889. Line 2 is the amount of HSA contributions that you made directly to the HSA, that is, not through your employer. Line 3 is the start of the calculation of the annual HSA contribution limit. This calculation is confusing, even by (or because of) reading the instructions, so if you have specific questions, you can ask them here, otherwise, "declare victory" and continue with your return.
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