よ is put at the end of a sentence when the speaker tells the listener something the listener does not know or has not noticed. It is used to give information, advice, or a warning, or to tell someone not to worry. It means something like 'you know' or 'I'm telling you', but it is often not translated into English.
§1Structure
Sentence + よ. Polite style: 〜です / 〜ます + よ (おいしいですよ, 来ましたよ). Plain style: verb / い-adjective + よ (行くよ, 熱いよ); な-adjective / noun + だ + よ (便利だよ, 休みだよ). Do not put だ after an い-adjective.
§2Examples
あ、バスが来ましたよ。
あ、バスがきましたよ。
Oh, the bus has come.
この本はおもしろいですよ。
このほんはおもしろいですよ。
This book is interesting, you know.
あのレストランは安くないですよ。
あのレストランはやすくないですよ。
That restaurant isn't cheap, you know.
「リンさんはどこですか。」「教室にいますよ。」
「リンさんはどこですか。」「きょうしつにいますよ。」
"Where is Lin?" "She's in the classroom."
きのうのテストは難しくなかったですよ。
きのうのテストはむずかしくなかったですよ。
Yesterday's test wasn't difficult, you know.
だいじょうぶですよ。
だいじょうぶですよ。
It's all right. (Don't worry.)
そのコーヒー、熱いよ。
そのコーヒー、あついよ。
That coffee is hot! (Be careful.)
あしたは休みだよ。
あしたはやすみだよ。
Tomorrow is a day off, you know.
§3Common mistakes
- Using よ for something the listener can already see or feel. When you and your friend are both looking at the rain, ✕「雨ですよ。」 is strange. Use ね to share the feeling: 「雨ですね。」 よ is fine when the listener has not noticed the rain.
- Putting よ on every answer. よ is strong, so with a teacher or a boss it can sound pushy. When the teacher asks 「宿題をしましたか。」, 「はい、しましたよ。」 can sound like 'Of course I did!'. The simple answer 「はい、しました。」 is safer.
- Adding だ after an い-adjective in plain style: ✕「おいしいだよ」 → 「おいしいよ」. だ is used only after な-adjectives and nouns: 「便利だよ」「休みだよ」.
§4How it's different from similar grammar
- 〜ね (agreement, confirmation)
- よ gives the listener information they do not have. ね is for information that both people share. 「この映画はおもしろいですよ。」 is said to someone who has not seen the movie (a recommendation). 「この映画はおもしろいですね。」 is said to someone who is watching it with you or has also seen it (asking for agreement).
- 〜よね (checking something you are fairly sure of)
- よね is よ + ね. The speaker believes something and checks that the listener thinks the same: 「あしたは休みですよね。」 (Tomorrow is a day off, isn't it? I think so.) With only よ, the speaker is sure and simply tells the listener: 「あしたは休みですよ。」
§5How this shows up on the JLPT
In N5 Grammar Mondai 1 (choose the right form), よ is one of the choices for a blank at the end of a line in a short dialogue, together with ね, か and other particles. Check who knows the information: if only the speaker knows it and is answering or telling the other person, choose よ. In Listening (choose what to say in the picture), sentences that warn or tell someone something, such as 「電車が来ましたよ」, often end in よ.
§6Quick check (3 questions)
1. Your friend wants to go to the library today, but does not know that it is closed. You tell your friend. Which sentence do you say?
2. Choose the word for the blank: A「このかさはだれのですか。」B「田中さんのです＿＿。」
A「このかさはだれのですか。」B「たなかさんのです＿＿。」
3. A friend is about to drink tea, and you say 「そのお茶、熱いですよ。」 What does よ add?
そのおちゃ、あついですよ。