Learn Finnish: Vastaatko sä mulle?

sä is the very common spoken form of sinä (you). mulle is the spoken form of minulle (to me). In standard written Finnish, this exact sentence would be Vastaatko sinä minulle?.

Why is 'to me' translated using the allative case (mulle)?

The verb vastata (to answer) always requires the allative case (the -lle ending) for the person you are answering. In Finnish, you literally say 'answer onto me'.

Can I say Sä vastaatko mulle? instead?

Not usually. When forming a yes/no question, the word with the -ko or -kö particle must jump to the very front of the sentence. Since you are asking about the action of answering, Vastaatko has to go first.

How do we know this means 'Will you answer me?' instead of 'Are you answering me?'?

Finnish does not have a separate future tense. A present-tense verb like vastaat can mean 'you answer', 'you are answering', or 'you will answer'. The context usually makes it clear that you are asking about a future action.