Learn Turkish: Mühendisin maaşı çok iyi. - The engineer's salary is very good.

The genitive suffix, which acts like 's, takes the form -in, -ın, -un, or -ün after a consonant. Because mühendis ends in the consonant s, we don't need the buffer letter n that you would use if the word ended in a vowel. We just add -in.

Why is there an -ı at the end of maaşı?

In English, we just say 'the engineer's salary'. But in Turkish, when you put a genitive 's suffix on the owner (mühendisin), you must also add a possessive suffix to the thing they own. So maaş (salary) becomes maaşı (his/her salary). Literally, the phrase works like 'the engineer's his/her-salary'.

Both mühendisin and maaşı get suffixes, so why do they use different vowels (i vs ı)?

They both follow 4-way vowel harmony, but they look at the last vowel of their own base words. The last vowel in mühendis is i, so its suffix gets an i (-in). The only vowel in maaş is a, so its suffix gets an ı (-ı).

There is no verb here, so how does the sentence mean 'is very good'?