Basic Grammar - の particle as an explanation


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 ?




> 「[飲]{の}むんだ。」

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.

> 「飲まないと[眠]{ねむ}れないから **飲むんだ** ！」

=

> "I drink (alcohol) because I couldn't sleep if I didn't drink!"

