～はずだ（đương nhiên）
Of course ... (no wonder)
Structure
[N/V/A] form that modifies nouns + はずだ
Explanation
Reasonably infers from cause to the conclusion 'naturally/obviously...'
Examples
1ドアが開かないはずだ。かぎが違っていた。
The door wouldn't open. The key was the wrong one.
2体がだるいはずだ。熱が３８度もある。
No wonder you're feeling sluggish—you have a fever of 38°C.
～はずだ（kết quả khác dự đoán）
Should have ... (but in reality ...)
Structure
[N/V/A] form that modifies nouns + はずだ
Explanation
Describes a result that differs from initial prediction or expectation
Examples
1汽車は３時に発車するはずだったが、急病人が出たために１０分ほど遅れた。
The train was supposed to depart at 3, but it was delayed about ten minutes because a passenger became ill.
2「変だなあ。このボタンを押せば動くはずなのに。。。」
That's odd. This button should make it work...
3日本へ来れば日本語なんかすぐに話せるようになると思っていた。こんなはずではなかった。
I thought that if I came to Japan I'd be able to speak Japanese quickly. It wasn't supposed to be like this.
～はずだ（nhớ chắc là…）
Be sure that ... but the reality is different
Structure
V た + はずだ
Explanation
Describes something the speaker is sure is true but in fact is different
Examples
1はさみが見あたらない。さっき見たときは、机の上にあったはずなのに。
I can't find the scissors. They were on the desk when I saw them a moment ago.
2バッグに入れたはずのケータイがない。どこかで落としたのだろうか。
The phone I thought I put in my bag is missing. I wonder if I dropped it somewhere.
3何度も確かめたはずだったのに、メールアドレスを書き間違えていた。
I could have sworn I checked it many times, but I had written the email address incorrectly.
4自転車をとられた。かぎはかけておいたはずなのだが。
My bicycle was stolen. I thought I had locked it, though.
Note
1) Easily confused meanings of ～はずだ. → Can mean 'naturally/supposedly' (reasonable inference), 'should have...' and also 'remember for sure'.2) Look at context. → If there is a basis for inference: 'naturally/supposedly'. → If the result contradicts expectation: 'should have...'.3) Easily confused with ～に違いない. → ～はずだ is based on reasonable inference; ～に違いない is a firmer judgment based on intuition/evidence.