had/did

#1 is correct; #2 is not.

Had Susan ever studied Thai before she moved to Thailand? (past perfect tense = past of "have" + past participle of the main verb)
Here is the answer using the past perfect:
Yes, Susan had studied Thai for several years before she moved to Thailand.

Did Susan ever study Thai before she moved to Thailand? (simple past tense. the main verb is used in its base form).

We might say Did Susan ever study Thai before she moved to Thailand? but I wouldn’t recommend writing it. This is a job for the past perfect, in my opinion.

I disagree. In this case, the simple past is better.

Susan always wanted to live in Thailand. So she studied Thai for several years before she moved there.

I think it’s the word ever that’s making me lean toward the perfect aspect. The question is similar to Has Susan ever studied Thai? except that the time period in the original continues up to the time at which she moved to Thailand rather than the present moment, in which case I wouldn’t say Did she ever study Thai?

Does that make sense?

Your second sentence is wrong grammatically. If you want to show to action use Past perfect. We can't use past indefinite. The first sentence is absolutely correct.

Yes it make sense I understand now. Thanks

Grammatically (except for 'studied' in the original) both forms are correct and in each the meaning is well maintained. However, in terms of usage we are talking about two related actions in the past. Susan studied Thai and moved to Thailand. The past simple is used for completed actions (usually only one) in the past when a specific time is mentioned. The past perfect is used to relate actions th

Had Susan ever studied Thai before she moved to Thailand?
Did Susan ever study Thai before she moved to Thailand?

Has Susan ever studied Thai? This only works without the subsequent past action of moving to Thailand.