Concluding from Reason

First of all, compare these two English sentences.

• He should have arrived by now.
• You should quit smoking.

In the first sentence, you are drawing a tentative conclusion; in the second, you are giving advice or a recommendation to someone. In Japanese, this distinction is important as it determines the appropriate word we need to use to express “should” in English.

Logical Expectation (はず)

はず indicates a conclusion based on your current knowledge of the situation, or a result that you believe is the natural, logical consequence of events.

【Verb Plain Form】はず

彼かれは既すでに到着とうちゃくしたはずです。

He should have arrived already.

赤信号あかしんごうを無視むしするのが違反いはんだと知しっているはずだよ。

You ought to know that running a red light is illegal.

In the above sentence, you are presuming that such knowledge is common sense.

彼女かのじょはさっき帰かえってきたので、部屋へやにいるはずです。

She just came back a moment ago, so she should be in her room.

締切しめきりが先週せんしゅうの火曜日かようびだったので、もう提出ていしゅつしたはずです。

The deadline was last Tuesday, so they should have submitted it already.

Connecting Adjectives and Nouns

While verbs connect directly to はず, other parts of speech require specific particles. This is a very common test question on the JLPT.

• い-Adjectives: Connect directly (e.g., 高たかいはず / It should be expensive).
• な-Adjectives: Require な (e.g., 便利べんりなはず / It should be convenient).
• Nouns: Require の (e.g., 日本人にほんじんのはず / They should be Japanese).

Modifying Nouns (はずの)

If you want “はず” to modify a noun that comes after it, the possessive particle の is required.

【Verb】はずの【Noun】

聞きかなくても知しっているはずの事ことだよ。

That is something you should know without having to ask.

The Strong Negative (はずがない)

Crucial JLPT Rule: To express that something “should not” be the case, we attach がない to the end. This translates to “There is no way that…” or “It is impossible that…”

あんなに真面目まじめな学生がくせいが、授業じゅぎょうをサボるはずがない。

There is no way that such a serious student would skip class.