I believe you technically can, but to be on the safe side I wouldn't. End it with something like "Sincerely, ___" or another similar phrase. Informal letters are pretty flexible depending on who you're writing to, but if this is for a test or something of the sort I'd play it safe.
Sorry, I disagree with the above answer, you would only use Sincerely in a FORMAL letter. For an informal letter you should use either: from, or love, as these are more endearing and informal
The type of pronoun is used to show the connection between the antecedent and the subordinate clause of a sentence is a relative pronoun. Relative pronouns are used to link a clause or phrase to a noun or pronoun. Hope this helped!
