Learn Turkish: Acaba yağmur yağar mı? - I wonder if it will rain?

While the future tense (yağacak mı?) asks about a specific event like "Is it going to rain later?", the aorist tense (yağar mı?) is often used for guesses, possibilities, or wondering about the future. Paired with acaba (I wonder), the aorist gives a speculative feeling: "Do you suppose it might rain?"

Why do we say yağmur yağar instead of just using a single verb for "to rain"?

In Turkish, the verb yağmak doesn't specifically mean "to rain"; it just means "to fall from the sky in large amounts" (precipitation). Because of this, you must always specify what is falling. You literally say yağmur yağar (rain falls) or kar yağar (snow falls).

How exactly do I use acaba, and does it always go at the beginning of the sentence?

Acaba means "I wonder" or "I wonder if". It doesn't change the verb conjugation, but it is almost always used in a question sentence (with a question particle like mı or a question word like "who"). It usually goes at the beginning of the sentence, but you can also put it at the very end for a slightly more thoughtful tone: Yağmur yağar mı acaba?

Why is the question particle mı here instead of mu, since the subject yağmur has a "u"?