I'm an absolute Japanese learning beginner. I came across this confusing sentence while I was reading a grammar guide book. Please help me : 飲むんだ。 I understand that の is substituted by ん as we speak, but I'm not sure what's the exact meaning of the whole sentence. "I drunk" or is it only an adjective and if so, what does it mean ?
Unless the context proved otherwise, that would be a request/order. It would be masculine speech nearly 100% of the time.
"Drink it up!"
The "other" main usage of 「～～のだ/んだ」 is when you explain (rather assertively) the reason or cause for something.
"I drink (alcohol) because I couldn't sleep if I didn't drink!"
I just replied today to a question that I believe might concern the same topic you are being confused by (since you don't give much context is a bit hard to say for sure).
Anyway, if I'm correct it might be an interesting reading: