Learn Hardcore Japanese: watasi ha mizu dake nomimasu.

は is the topic marker. It tells the listener “this sentence is about me.” Unlike the subject marker が, は sets 私 as the topic and can carry a slight contrast (“as for me…”).

What does だけ mean, and why is it placed after 水?

だけ means “only” or “just.” When you attach it to a noun—in this case 水—it restricts the meaning to “nothing but water.”

Why is there no を after 水 or 水だけ?

With だけ, the object marker を is often dropped for simplicity. You could say 水だけを飲みます, but native speakers usually omit を when だけ is present.

What does 飲みます tell us about tense and politeness?

飲みます is the polite non-past form of 飲む (“to drink”). Non-past covers both present and future, so it can mean “I drink only water” (habitual) or “I will drink only water.” The ‐ます ending makes the statement polite.

Could you drop 私は and just say 水だけ飲みます?

Yes. Japanese often omits the topic when it’s clear from context. 水だけ飲みます is more direct or casual. Including 私は adds emphasis or contrast (“as for me…”).

Can だけ be used with other parts of speech besides nouns?

Yes.

With verbs (dictionary form): 見るだけ (“just look”)
With na-adjectives: 静かなだけ (“just quiet”)
With i-adjectives: 高いだけ (“just expensive”)
However, when used with verbs or adjectives, だけ often implies “nothing more than…” rather than a strict “only.”