Learn Croatian: Zimi često moram napumpati gume. - In winter I often have to inflate the tires.

While you can say u zimi, zimi is a very common adverbial form that directly translates to "in winter" or "during winter". Croatian frequently uses these specialized adverb forms for seasons, like ljeti (in summer) or jeseni (in autumn), instead of using a preposition.

The word često (often) usually goes with imperfective verbs. Why is the perfective verb napumpati used here?

This is a great catch! The word "often" applies to how frequently the whole situation happens (the obligation to inflate). However, on each of those individual occasions, the action of inflating the tires is a completed event. If you used the imperfective pumpati, it would sound like you spend your winters continuously in the ongoing process of pumping air.

How does the prefix na- change the meaning of the base verb pumpati?

The base imperfective verb pumpati just means "to pump" in general. Adding the prefix na- makes it perfective (napumpati) and adds the specific meaning of filling something up until it is full. This is a common way Croatian creates perfective verbs to show that a physical task has reached its intended conclusion.

Could I say moram da napumpam instead of using the infinitive napumpati?