正しい文法

No offense, but you might be mistaken. This Q&A site is for questions on English. Not Japanese.

That said, I would like to help, but I honestly do not know the answer to your question. I can put the numbers in the right order (2-1-4-3), but the sentence still needs a word before が.

手紙を　出す前に　(…) が　できます。

(You/we) can (…) before sending a letter.

I have no idea what (…) would be.