<p>So there are really a few different answers to this question.</p>
<p>Generally speaking, there's some, but not total, flexibility in class choice. Beyond freshman year, it depends on your major; some majors are very free-form and just ask that you take a certain number of classes in the department, while other majors require a standard set of classes.</p>
<p>As far as the GIRs go, there are several options available to fulfill each requirement. For example, the single-variable calc requirement can be fulfilled by taking 18.01 (regular differential/integral calculus), 18.01A (for people who took AP Calc AB in HS; 6-week review of differential/integral calc, then on to multivariable), or 18.014 (differential/integral calc with theory). I do think most people have at least seen calculus before coming to MIT, so it might be a little tougher for you, but certainly not impossible if you're willing to work hard.</p>
<p>(I'm not sure if this is what you mean--) There's also a pretty extended period of time where you can add or drop classes without penalty. This year, school started Sept 7, but we're allowed to add classes to our schedules until Oct 7, and we're allowed to drop classes from our schedules until Nov 23. That way, if you sign up for a class which you quickly realize is too difficult, you're free to switch it for a class which suits you better.</p>
<p>However -- there's not, ideally, a huge amount of adding and dropping classes first term freshman year, since there's not much you can take if you don't have the basic GIR classes under your belt.</p>