This phrase is NOT the infinitive form. "Se chover" is using the future subjunctive case. The future subjunctive often looks like the infinitive form, but that is only so for regular verbs. You can notice the difference with irregular verbs like in the phrase "se vocês tiverem arroz, vou comê-lo" "if you have rice, I will eat it"
Oh okay that makes sense. The subjunctive is a mood we use when we are not certain of the probability, yes? (IF it rains.. I don't know if it will rain but IF it rains.)
Is it a good rule of thumb to just assume the future subjunctive matches the infinitive a good 60-70% of the time? (with regular verbs of course)