Learn Turkish: Şu mühendisler kim? - Who are those engineers?

In Turkish, adjectives never take a plural suffix. When a demonstrative word like şu is used right before a noun to point it out, it stays singular. Since the noun mühendisler is already plural, adding a plural suffix to şu is unnecessary and incorrect.

How do we know to use -ler instead of -lar for mühendis?

To decide between -lar and -ler, we look at the last vowel in the word. The last vowel in mühendis is i. In the 2-way vowel harmony system, front vowels (e, i, ö, ü) always take the -ler suffix.

Why is the question word kim (who) singular if we are asking about multiple engineers?

Because the subject mühendisler is already explicitly plural, the question word does not need a plural suffix. While you can sometimes say kimler to emphasize a group, simply using kim is the most common and natural way to ask this.

Where is the word for 'are' in this sentence?

Turkish does not have a standalone word for 'am', 'is', or 'are'. In simple present tense sentences like this one, the 'are' is simply implied by the context. The sentence literally translates to 'Those engineers who?'