Your two short sentences are fine. You are right to think that 'quench' collocates with 'thirst'. Your long sentence is perfectly correct in terms of meaning and grammar - it's just slightly awkward. We'd tend to say something simpler, along the lines of 'I wasn't thirsty any more'. In fact, 'quench' isn't used very much in everyday conversation, and we'd be more likely to say something very ordinary such as 'I didn't need any more to drink, so I gave the rest of my water to my friend' rather than the more formal and old-fashioned/dramatic-sounding 'My thirst was quenched'. That's more like 'Game of Thrones' kind of language :)

Yes, you are using it perfectly. It is an old word that we do not use very much. It is still used, at least in Australia, but mainly in product advertising (e.g. for beer or electrolyte drinks) or for a very emphatic expression indicating that the thirst is or will be completely gone (like the other usage of the word, meaning to extinguish a fire completely). Some other examples would be: (Product X) quenches your thirst, fast "thirst quenching" Quench that thirst (with Product X) Wow, that drink really quenched my thirst! (this is a bit weird and not really something that I would say except maybe ironically in a pub). In general conversation with native-speaking friends, I would probably say something like "I am really thirsty" or I was really thirsty until I drank that", avoiding the use of the word. It is not incorrect to use it and we do use it, but it is a bit formal and old-fashioned. It is more likely to be used between educated people and in product advertising that suggests that the associated drink is very good at fixing your thirst. I hope that this is helpful. It is an interesting word. EDIT: I think that part of it is that I would rarely talk about the need to get rid of thirst! The word is perfectly ok, it is just not something that you use very often. Maybe it is because I don't talk about thirst very much. ;) EDIT 2: Andrew Goddard's comment above is also correct, I did not see the question in the heading "When I bought a bottle of water, my thirst had already been quenched". I don't think that this sentence is grammatically incorrect but it is a bit unusual. Why would you buy water when your thirst had already been quenched? It is a bit like saying : "I was already full when I ordered the meal". It is a possible situation but it is unlikely to occur.

No, this sentence is not correct. The sentence uses the past perfect tense. We use this tense when two different things have occurred, one after the other, but where both actions are in the past. So somebody might say: "Before I drank a bottle of water, I had been dying of thirst." Your sentence, above, means that you bought a bottle of water even though you were no longer thirsty, which does not seem to make much sense. But the biggest reason that the sentence is incorrect is that it does not describe two things, both of which are in the past, that happened one after the other. You could say: "Although my thirst had already been quenched, I decided to buy another bottle of water anyway because it was such a hot day."

Generally speaking a better way to pose this question might be to say, It is so hot outside that I need water to quench my thirst.