<p>Other than intro language courses, very few classes are taught primarily by graduate students. In large classes, however, a professor gives the lecture portion of the class and the small discussion groups that meet once a week are lead by TAs.</p>
<p>I’ll let a more current student answer the question about English language abilities. There were a couple of notoriously incomprehensible math and engineering TAs in my day, but not many. The vast majority of TAs were great.</p>
<p>At Yale, all professors have to teach an undergrad course every year. This means they can’t lock themselves away with their grad students. Yale is noted for its undergrad focus in part because of this–you don’t come to Yale unless you want to teach undergrads.</p>