In no circumstance should the plural they be used for a singular case of either he or she. All people should know this. Use the word which only in a prepositional phrase or when set off with a comma, which is the correct way to use the word. Use that in all other cases. Effect and affect are not the same word.
Contractions have no place in formal communication.
Please do not use them in your papers. Do not end sentences with prepositions. This is a mistake with which I care not to deal. Do not be led astray by typing lead. A good paragraph is indented and should have a minimum of three sentences.