Using an op amp as conditional
You can use an opamp as a voltage comparator. Without feedback, the output will basically stick to the positive or negative power rail according to the sign of the voltage between IN+ and IN-. All the opamp does is amplify the voltage between its inputs by "a huge amount of gain". 

It won't be a good comparator: in this role, opamps are slower, the output might not get close to the power rails (depending on the opamp) but it'll work. If all you need is to light a LED, then no problem.

Note this homework will teach you bad habits. You shouldn't use opamps as comparators unless you have a dual opamp somewhere in the design, and one opamp is unused, and you can tolerate the bad performance. If you want a fast comparator (like &lt;1µs) then use a comparator.