If you buy the movie tickets_______I'll buy the treats.

Question

A) than

b) then

C) both depending on context.

Answer 1

It's an "if, then" sentence. Answer is then.

Answer 2

Than is only used for comparisons:

He is older than his sister.

It is warmer in Florida than it is in Maine.

The bus holds more people than the car does.

So the question to ask is is this sentence making a comparison?