if i don't answer part of this right, is it a problem?
No, I don't think so.
I believe they just want to know about any other names in order to check your previous travel history etc. under the other name.
They will have your old passport, new passport, and your marriage certificate, so it will be obvious what the timeline of events was in regards to changing your name.
If you legally changed your name, when did you do this (if applicable)?
15 December 2020 this is the issue date on my new passport that has my married surname on it
Have you ever entered the UK using this name?no i can't make out if they mean my married or maiden surname
It reads to me as if they mean the new name, because it's in the same section of questions about the name change:
"Why did you change [to the new name]?
When did you change [to the new name]?
Have you ever entered the UK using [the new name]?"
If you're worried about it though, you can just include a cover letter explaining that you weren't sure how to answer that question.