How long had you been studying English before you started speaking fluently?
How long did you study English before you started speaking fluently?
What's the difference between the two meaning-wise?
Most important, which contexts are appropriate for each?
There is no difference in meaning.
Native speakers of many Indo-European languages have trouble with perfect tenses in English. In French, German, Latin, and Spanish for example, the proper tense for past time is most often the present perfect. This is not true in English. There, the proper tense for past time is most often the simple past. The present perfect has several meanings in English, but it cannot be used generally for past events.
Based on the other Indo-European languages I know, English uses the past perfect in the same way as they do: the past perfect is used to indicate a past action that preceded some other past action. In English, however, the past perfect is not mandatory if other words like “before” or “after” make the sequence clear.
He was fluent in English after he had studied it for over six years
He was fluent in English after he studied it for over six years.
Both mean the same thing, and both are idiomatic. People who are very careful in their usage will usually use the past perfect, but, in casual writing and most colloquial speech, the simple past will be used frequently. Notice, however, that “after” makes the sequence explicit.
Yes, that speech was in flawless English, but remember that he had lived in Britain for a decade as a child.
Here, the past perfect is mandatory because only tense is available to specify sequence.
