What is the way that you have to write a sentence correctly?

It depends on what you're asking. Are you asking grammar wise or punctuation wise? There are many different ways to write a sentence correctly and English is one of the most difficult languages; that means that there are many loop-holes with sentence making.

Subject, verb, & be a complete thought

Use who. if it wasn't a question, you could put the he in the blank.  if you can put he or she in the blank, it's who.if you can put him or her in the blank, it's whom.