Learn Turkish: Niçin meşgulsün?

This is a famous exception to the vowel harmony rules! Because meşgul is a borrowed word from Arabic, the 'l' at the end is pronounced softly, closer to the front of the mouth. Because of this soft pronunciation, the word takes front vowel suffixes like -sün instead of the expected back vowel suffix -sun. Another common word that breaks the rule in this exact same way is alkol.

What exactly does the -sün suffix do here?

It is the 'you are' ending for a single person. In Turkish, there is no separate verb for 'to be' (like 'am', 'is', or 'are') in the present tense. Instead, you attach personal suffixes directly to nouns or adjectives. Here, meşgul means 'busy', and adding -sün turns it into 'you are busy'.

Can I use neden instead of niçin to say 'why'?

Yes, absolutely! Both niçin and neden mean 'why' and are usually interchangeable in everyday conversation. Niçin is actually a combination of the words ne (what) and için (for), literally meaning 'for what'.

Do I need to put sen (you) at the beginning of the sentence?

No, it is not necessary. The -sün ending on meşgulsün already makes it perfectly clear who the subject is. You could say Sen niçin meşgulsün?, but Turkish speakers almost always drop the sen unless they want to add special emphasis to the word 'you'.