First, is my question right? Does amongst fit here?
Please differentiate the above phrases.
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I have used cocaine.
I took cocaine at least once sometime in the past.
I was using cocaine.
In the past, I was a habitual user of cocaine. EDIT: As the comment says, this can also mean a process in the past, e.g. "I was using cocaine when the accident happened" can mean "I was not looking at the road since I was snorting cocaine."
I have been using cocaine.
Starting some time in the past, and including today, I use cocaine.
I had used cocaine.
With my reference point at a time in the past, by that point, cocaine was already taken by me. Better example:
By the time I met my girlfriend, I had used cocaine.
So when meeting the girlfriend, you took cocaine by then already.
BONUS: You missed one:
I had been using cocaine.
Meaning, with a reference point in the past, starting a time before then up to the reference point, I was habitually using cocaine up to and including that point.
Why not put in some other wonky tenses?
I will have used cocaine.
I will have been using cocaine.
Here is a link conjugating it in all its tenseful glory.