"Da komme ich gerne mit" what is the best translation for "da"
Something like "in that case", "if this is so", "under such circumstances" or just plain "so"/"then" :-)
Man kann dort im Freien sitzen.
Da komme ich gerne mit – ich sitze gerne im Freien.
There, you can sit in the open air.
Then (if so, in that case) I'd like to go along: I like sitting in the open.
"Da" is a tricky word. I'm sure you're aware of its basic meaning of "there". It is also often used in the above sense. As in the primary meaning, it is an adverb, which explains the structure of Da + verb + subject (V2).
It can also be a conjuction, meaning "since", "because", "given that":
Da ich keine Lust habe, komme ich nicht mit.
Since I'm not feeling like it, I'm not coming along.