Learn Turkish: O, çiğ soğanı yemezdi. - He didn't use to eat the raw onion.

It combines the verb root ye- (eat) with the aorist negative suffix -mez and the past tense suffix -di. The aorist tense is used for general habits, so adding the past tense creates a past habit meaning "used to" or "would".

What is the difference between yemezdi and yemedi in this context?

Yemedi is the simple past negative (He didn't eat), which refers to a single, specific event. Yemezdi uses the aorist past negative to show a habitual or repeated action in the past (He didn't use to eat or He wouldn't eat).

Why is the accusative case used on soğanı here?

The suffix -ı makes the onion a specific direct object, meaning "the raw onion". If the sentence were about his general habits with onions, it would drop the suffix and just be çiğ soğan yemezdi (He didn't use to eat raw onions).

Why is there a comma after O?

The comma separates the subject pronoun O (he) from the rest of the sentence. Without it, O çiğ soğanı could easily be misread as "That raw onion", since o is also the pointing word for "that".