what is the gramatical and "meaning" difference between I drank last night and I had drunk last night?

Answer 1

"Had drunk" is incorrect. "Drunk" is a state of being. "Drunk" is often used as an adjective or adverb.

"I had drunk" may be correct, but it is very clumsy and the thought can be expressed more gracefully. See my example sentence below.

"I drank" is a verb phrase, meaning the subject "I" had consumed a liquid beverage the previous night.

"I drank so much last night I became very drunk."

"Drank" is the verb. "Drunk" is an adjective.

Answer 2

I drank last night is simply written in the past tense. I drank.
If you meant to write, "I got drunk.", that would mean you drank enough to become inebriated.
However, "I had drunk last night .", is not incorrect. This form indicates that you engaged in the act of drinking last night, and now you are describing what happened after the drinking was finished.

You could write, "I have drunk water all my life." This indicates you have drunk water and continue to do so. You might also say, "I had drunk water all of my life until they put in the dam. Then the water became polluted." You use "had drunk" because the drinking of water stopped.

Answer 3

Grammatically, the sentence "I drank last night," is an example of the simple past of "to drink" with the addition of the time adverbial "last night". The sentence "I had drunk last night," is an example of the past perfect tense of "to drink" again with the time adverbial "last night".
The difference in meaning is in the degree of relevance "last night" has. With the simple past, "last night" is likely to refer directly to the night before. It would likely come up in direct speech. However, with the past perfect, "last night" is less important, almost a side issue. This would most likely come up in indirect speech, as in "He told me he had drunk last night."