The Verb 喝(hē)
Unlike its English counterpart, 喝 always functions as a transitive verb. In other words, unless it's clear from the context, the beverage has to be specified. Therefore, 他常常喝 is not a complete sentence unless the beverage is understood.
A: 他常常喝咖啡吗?
B: 他常常喝。
A: 他常常喝咖啡吗?
B: 他常常喝。