If you want to be correct, smultron is, according to Wikipedia, wild strawberry, woodland strawberry, Alpine strawberry, European strawberry, or fraise des bois. It seems there's some overlap though, so that people may refer to smultron as just 'strawberry', so smultron is still an accepted answer here.

It's called the 'formal subject', and it's always 'det', no matter what the gender of the 'real subject'. In English we write it as 'it' or 'there' - so for example, "A man is in the kitchen/En man är i köket" becomes "There is a man in the kitchen/Det är en man i köket". "Det" is the formal subject, "en man" is the real subject. It's worth looking up for a better understanding, but by and large whenever 'it' or 'there' takes the place of the subject and pushes it later on into the sentence, the Swedish translation is 'det'.

Can't explain why it's like that, just learn that most of the time when it's "it is ...", then it's "det är ...".

Vad är det [där]?

Det är en/ett ...

I'm not sure why, but that's correct (should have paid more attention in Swedish class). My best guess is if you ask "what is that" then the answer begins with "it is..." no matter what the item is. It is then followed by the item, in this case "... a strawberry." In Swedish, the question would be "vad är det där" so the answer is "det är... en jordgubbe."