Questions & Answers about Ja, er trinkt oft Wasser.
Does Ja count as the first word for the verb position rule?
No. Words like Ja (yes) and Nein (no) are set off by a comma and sit outside the main sentence. They take up "position zero." The actual sentence starts with er (position 1), so the verb trinkt safely stays in its normal second position.
How would I ask the yes/no question that this sentence is answering?
You would use Verb-first word order! To turn this statement into a yes/no question, you simply take the verb from the second position and move it to the very front: Trinkt er oft Wasser? (Does he often drink water?).
In English we say "often drinks," so why does oft come after the verb here?
In a regular German statement, the verb must be in the second position. Since the verb is strictly locked into that number two spot (er trinkt), adverbs like oft (often) usually have to go after the verb, rather than before it.
Why does the verb end in a -t here (trinkt)?
Because the subject is er (he). The dictionary form of the verb is trinken (to drink). When talking about er, sie (she), or es (it), we drop the -en ending and add a -t.