"I am drinking his beer."
Translation:Mi trinkas lian bieron.
I think it should be "drinkas" because the person is drinking an alcoholic beverage.
Drinki has two meanings.
Drinking alcoholic beverages
When drinki comes in that sense it DOES NOT have a direct object (intransitive verb).
Drinking excessively with intention of having fun
When it comes in that sense, it may have a direct object (transitive verb).
Maybe it's for the -ing verb context? I came looking for the same answer, so it's just a guess at this point
This is covered later in the course. "Mi estas trinkanta" is the present progressive tense of the verb. It means "I am drinking" as opposed to "Mi trinkas" which is in the simple present -"I drink". In general conversation, we usually use the simple present tense, unless we really want to emphasise that the action is happening right now, at this very moment. English tends to use the present progressive a lot more than Esperanto.
REALLY!? first I wrote this (down below!) and gimme error finally I wrote trinkanta now it say oh there is another correct solution... just because duolingo free it does not have to be annoying...
Another correct solution: my bum.... I wrote this and gimme error now accepting it. ?!?!?!?!?
Mi trinkas lian bieron.
No. We put an -n on the ends of words when they are the object of the sentence. "Lian bieron" is the object of "trinkas", so the -n is required. You can read more about this in the Tips & Notes for Accusatives: