Learn Turkish: Bebek çabuk büyüyor mu? - Is the baby growing fast?

We take the present continuous verb büyüyor (is growing) and add the question particle mu after it. Remember that question particles are always written as a separate word, even though they belong to the verb.

The verb is 'büyümek'. How did it become 'büyüyor'?

We drop the -mek to get the root, büyü- (grow). Because büyü- already ends in a vowel, we don't need to add a connecting vowel. We just attach the continuous suffix -yor directly to get büyüyor.

Why is the question particle mu and not mü? The verb root has the vowel 'ü'.

Vowel harmony always looks at the very last vowel of the preceding word. The last vowel in büyüyor is the o in the -yor suffix. According to 4-way vowel harmony rules, an o or u is always followed by mu.

Does çabuk mean 'fast' or 'quickly' here?

It means both! In Turkish, adjectives like çabuk (quick/fast) can often be used directly as adverbs (quickly/fast) to describe an action, without needing to add a special ending like '-ly' in English.