What it means
だけ marks a limited amount, person, thing, or action and means “only” or “just.” It commonly follows a noun or a plain-form clause. The rest of the sentence can still be familiar or polite, so だけ itself does not determine the register.
Usage note
だけ usually works with an affirmative predicate. Do not confuse it with しか, which generally requires a negative predicate and emphasizes a stronger sense of limitation.
How to use it
NounだけVerb (ます-stem)ます
私は水だけ飲みます。
I drink only water.
Verb (plain form)だけです
私は本を読むだけです。
I only read books.
NounだけVerb (dictionary form)
私は水だけ飲む。
I drink only water.
Verb (plain form)だけだ
私は本を読むだけだ。
I only read books.
Example sentences
朝は水だけ飲みます。
In the morning, I drink only water.
週末は一時間だけ勉強します。
On weekends, I study for only one hour.
私は図書館で料理の本だけ借ります。
I borrow only cooking books at the library.
妹は旅行の日