"He is drinking a glass."
Translation:Il boit un verre.
Boire is the infinitive form of the verb: "to drink"
It is conjugated like this:
I recommend that you find and use a conjugation site. It is good practice to write out the most commonly used verbs until it comes naturally without thinking. Here's one website: http://la-conjugaison.nouvelobs.com/du/verbe/
The English sentence here is really awkward, in my opinion. Basically "il boit un verre" means "he's having a drink". I don't know why they chose to have the literal translation in the English to French exercise, but I know that "he's having a drink" is accepted as the English translation when you're given the French sentence.