Learn Turkish: Patron yeni sekreterden şüphelendi mi? - Did the boss suspect the new secretary?

In English, you 'suspect someone', but in Turkish, the verb şüphelenmek (to suspect) requires the ablative case (-den or -dan). You can think of it as getting your suspicion from someone. That is why we use sekreterden.

Is yeni sekreterden treated as one unit for the case ending?

Yes. Yeni (new) is an adjective describing sekreter (secretary). In Turkish, case endings only attach to the noun at the end of the phrase, so the -den suffix goes on sekreter, leaving yeni unchanged.

What are the exact parts making up the verb şüphelendi?

It starts with the noun şüphe (doubt). The suffix -len turns it into the verb şüphelenmek (to suspect). Finally, the -di suffix is the simple past tense for 'he/she/it', creating şüphelendi (suspected).

Why is the question particle mi placed at the very end of this sentence?

The question particle mi usually comes right after the word you want to question. By placing it at the very end after the verb, the sentence focuses on the action itself, asking specifically if the act of suspecting happened.