In English you would use *only*, but it Dutch you'll have to choose between *alleen, maar* and *pas*. Be aware of words of quantity.
Sometimes more than one option would be correct. Pick the most logical one.
1.Kevin is _____ zeventien, dus hij mag nog geen alcohol.
maar
alleen
2.Ik kan niet vaak op vakantie, want ik heb _____ twintig vakantiedagen per jaar.