Translation:He is drinking the alcohol; I am drinking the water.
This is a bit simplistic. Depends whether the infinitive of the verb ends in 'ir, re or er' PLUS there are the irregular verbs that don't fit the regular category. 'Boire' is one of the irregular verbs. AND, the ending of the verb is going to depend upon the tense. Right now, in the beginning, it's easy as we're dealing with present tense. But the endings are going to change for the other tenses. Duo 'snuck' a future in on me, using 'mangera'. Best thing to do, for anyone, is go to a used book store (or use amazon's used book feature) and get a basic french text book. It will show the conjugations, etc. All clear. Gosh I've got five texts, I've a book that conjugates 501 verbs, just received the 'French in Action' dvd's. I can't see relying on Duolingo alone to learn the language.
For boire specifically:
In French, sometimes you pronounce the "s" at the end of a word, sometimes you don't ( http://french.about.com/library/pronunciation/bl-liaisons.htm ). Usually, there is no difference in pronunciation between "il" and "ils", and you need to tell them apart either by the verb conjugation (il boit versus ils boivent) or by the context.
Well french has different translations for just one word so don't be confused. In english when you ask someone if the person drinks alcohol you'll ask you drink? Such in this case when you are talking for someone and yourself you say he drinks or he drinks alcohol. So in this case alcool will be alcohol direct no confusions.
Sometimes duolingo goes for a word for word translation and sometimes they don't. I guessed wrong the first time and put. He drinks alcohol, I drink water. Not sure which is a better way to learn or teach a language. When you translate word for word it might help you get into the syntax of a language. Or you might let the awkwardness get in the way. Maybe however it is better to learn comparing in your brain the best most currently spoken translation . I think the point is that you can't fit french into your english mold. And I'm not sure if word for word is better or not.
When I noticed that the first "l" here is pronounced as "ee", this is exactly what I did: I went to Google Translator, and it pronounces the first "l" as a "l" ("al-col", instead of "aee-col"). It sounds better to me as well, but I'm also not French; this is why I asked for confirmation here.
I don't know how it sounds on other people's computers, but to me it sounds like the sound is broken in the middle of the sentence, and it seems like there's an "et" to combine the two parts. Of course, that's what makes sense. Such sentences are not normally written with a semicolon in the middle.
"l'alcool" means "the alcohol". "de l'alcool" means "some alcohol" or just plain "alcohol". There always has to be an article in French. They don't say "I drink alcohol" like in English and other languages. So you can't say, "Je bois alcool". You have to say "Je bois l'alcool" if you mean "I drink the alcohol" and "Je bois de l'alcool" if you mean "I drink some alcohol" or "I'm drinking alcohol" which means the same thing as "I'm drinking some alcohol" in English. If you want to say, "I like drinking alcohol" in French, you'd use " l'alcool".
It's the way the French talk.
I put "he drinks alcohol, I drink water". I didn't think the definite article was necessary in English as we would tend not to say "the" in this context (would we?). Cross that it wasn't accepted but accept that if they wanted this answer they might have put "d'alcool" not "l'alcool" etc. But think it's a bit petty!