Learn Turkish: Savcı şüpheliyi sorgulamalıydı. - The prosecutor should have interrogated the suspect.

It is built on the verb root sorgula- (to interrogate). First, we add the necessity suffix -malı (should/must). Then, to show it was an unfulfilled past obligation ("should have"), we add the past tense suffix -dı, using the buffer -y- to connect the two vowels. Together, sorgula-malı-y-dı means the prosecutor should have done it, but didn't.

Doesn't şüpheli usually mean "suspicious"?

Yes, şüpheli often acts as an adjective meaning "suspicious." However, Turkish frequently uses adjectives as nouns. In police or legal contexts like this one, şüpheli acts as a noun meaning "the suspect."

Why does şüpheli have a -yi ending here?

The -i (which becomes -yi after a vowel) is the accusative case suffix. It is required here because "the suspect" is the specific, definite direct object receiving the action of the interrogation.

Where does the verb sorgulamak come from?

It traces all the way back to the basic verb root sor- (to ask). From there, it becomes the noun sorgu (interrogation or questioning). Finally, adding the suffix -la turns that noun back into an action verb: sorgula- (to conduct an interrogation).