Why do I use kaette instead of kaeru in the sentence "kinou wa uchi ni kaette nani wo shimashita ka."
The -te form can be used to connect two sentences, which implies a connection which may or may not be so obvious at first glance. As separate sentences, "Kinou ha uchi ni kaerimashita. Nani wo shimashita ka," would have no implied or explicit relationship. "You came home yesterday. What did you do?" The -te form makes it "When you got home yesterday, (and then) what did you do?"