What determines if a LingQ is "learned"?

That’s up to you! Some people mark them as learned when they can recall the target language word by looking at the clue. Some don’t mark them as learned until they understand them when heard in a lesson. Some mark them as learned when they are able to use them without looking them up. And some don’t study flashcards at all! They mark the word as learned when they see it in a new lesson and understand what it means without looking at the definition.

This topic is discussed periodically here. If you search the forum, you can probably find previous discussions. Enjoy your language learning.

Just to toss my thoughts into this discussion,

I have found that even words i thought i knew i don’t because i keep encountering different meanings (or different shades of meaning).

For example, I had the word “livre” (which means book in French) marked known. Just the other day, I came across another use of the word “livre” with regards to weight (ie. pounds/kilos). It is no longer marked as known.

I think a good approach is to not be extremely picky on setting words as known because if you do enough reading you will continually come across different meanings for words you already supposedly know. I made this mistake at the beginning; i no longer am so hard on myself.

My two cents