Because you know that it's either raining or snowing. You're not asking if "yağmak" is happening or not. You know for a fact that something is falling from the sky, but which is it? Is it Rain or is it Snow? That's why you question the "yağmur" and "kar" by placing the "mi" after those words.
However, isn't the fact that “yağmak” is a verb, more of a reason to place “mi” at the end? Does this type or rather, STYLE of emphasis occur within most sentences that focus on the noun(s)?
Ex.: Is is rain(ing) or snow(ing)?
parenthasises are used in order to represent an implied message here
I'll give one example. If somebody says 'It is raining.' to you would imagine about water drops falling onto the ground. And that sentence would be 'Yağmur yağıyor.' in Turkish. But if the same one said 'It is raining blood(for example)'. you would imagine about drops falling but this time blood drops instead of water. And that sentence would be 'Kan yağıyor.'
In conclusion, the verb 'yağmak' is about movement and doesn't contain the matter type. If you said someone 'Yağıyor.' without a conversation before, it would be probable that said someone asks you 'Ne yağıyor?'
Not in general meaning. That would be 'düşmek'. But for a sentence like 'Snow is falling.' you would translate 'fall' as 'yağmak'. But 'yağmak' is always used in that context: 'falling from above(mostly sky) in little pieces/amounts. If it is a natural phenomenon, little amount would be 'raindrops' or 'snowflakes' . But in a sentence like this:
'Üzerimize para yağıyor.'(It is raining money on us.)
little pieces would banknotes.
You look at the final vowel of the previous word. For Yağmur, it is "u".
If the last vowel is A or I, use "mı": kar mı, ışık mı
If the last vowel is E or İ, use "mi": dere mi, kil mi
If the last vowel is O or U, use "mu": yol mu, torun mu
If the last vowel is Ö or Ü, use "mü": göl mü, kül mü
Yes, you got it! :) Correct ones:
-> Yağmur mu, kar mı yağıyor?
-> Yağmur mu yağıyor, kar mı?
-> Yağmur mu yağıyor, kar mı yağıyor?
-> Yağmur mu yoksa kar mı yağıyor?
-> Yağmur mu yağıyor yoksa kar mı?
-> Yağmur mu yağıyor yoksa kar mı yağıyor?
NOTE: Actually you can see that one verb is optional in both two ways (both with yoksa and without yoksa)
EXTRA ONES WITH A DIFFERENT WAY TO EXPLAIN THE SAME VERB
-> Yağmur mu yağmakta yoksa kar mı yağmakta?
-> Yağmur mu yağmakta, yoksa kar mı?
-> Yağmur mu yağmakta, kar mı yağmakta?
-> Yağmur mu yağmakta, kar mı?