Can you get pregnant only when you ovulate?

No: Well sort of...You have to ovulate to get pregnant but you could have sex 7 days prior and the sperm can live inside you then you could ovulate and the egg could then be fertilized.

A qualified yes: Pregnancy always requires an ovulation. But since sperm can survive up to 6 days when conditions are favorable, you can potentially get pregnant if you have had sex as many as 6 days before you actually ovulate. Even if you know when you ovulate, as many women do, that does not give you sufficient warning to reliably allow you to avoid pregnancy.