<p>GreatScott, there are several different kinds of courses at Yale, which all work differently.</p>
<p>In the humanities and some social science classes, lecture classes will meet twice a week for lecture (given by the professor), with one more meeting for a smaller section run by a TA.</p>
<p>In the sciences and most social sciences, lecture classes will meet either 2 or 3 times a week (if twice a week, then for a longer time), possibly - though not always - with an optional section in addition - often used for going over problem sets, or the like.</p>
<p>Seminars generally meet either 1 or 2 times a week (if one, then for about twice as long) in a group capped somewhere between 15-20 (depending on the department) and always with a professor - there are no TAs in seminars.</p>
<p>Lower level language classes meet 5 times a week, always with a language instructor in classes with around 10 students. In addition, in some languages, you may have a weekly one-on-one session with a grad student TA for language practice (and possibly oral exams).</p>
<p>But the basic lesson is that all classes will spend most of their time being taught by a professor, and in any case, lectures are all given by professors (except on the rare occasions when they may ask one of their grad students to give a lecture in a particular area of expertise, probably related to their PhD thesis).</p>