Will vs Present Simple

When you make a decision at the moment of speaking (particularly in response to a request), use will. Do not use the present simple (or present continuous or going to).

will is typically used 1) to express a spontaneous decision or make an offer to do something (e.g. I’ll help you with your suitcase); 2) in emails and letters to refer to attachments or say what action we intend to take (e.g. I’ll get back to you with the information you requested); 3) to predict future events, based on personal intuitions (e.g. I think they’ll lose the election).

With when, before, after, until, if (i.e. words connected with time), use the present tense directly after the time word and will in the other part of the clause (e.g. When / If I see her, I will tell her).