Learn Turkish: Niçin ağlıyorsun?

Niçin and neden both mean "why" and can be used interchangeably in most situations. Niçin is actually a combination of ne (what) and için (for), literally meaning "for what".

The verb to cry is ağlamak (root ağla-). Why is it spelled ağlıyorsun with an "ı" instead of an "a"?

When a verb root ends in "a" or "e", that vowel changes to a narrower vowel ("ı", "i", "u", or "ü") when you add the present continuous suffix -ıyor. So, ağla + -ıyor becomes ağlıyor (crying) to make it easier to pronounce.

Why isn't the word sen (you) in this sentence?

Turkish often drops the subject pronoun because the verb ending already tells us who is doing the action. The -sun at the very end of ağlıyorsun is the personal suffix that specifically means "you" (singular).