N4
はずだ
Bound to (be), Expected to (be), Should be (able)
今日は雨が降るはずだ。
It's supposed to rain today.
Explanation
「はずだ」 has 2 uses
1Should, ought to be
Based on solid reasons or common sense, you're sure it's so: 「会議は2時に始まるはずです」 the meeting should start at two.
彼は借りたペンを返す筈だ。
He is bound to return the pen he borrowed.
今日は雨が降るはずだ。
It's supposed to rain today.
2No wonder
Suddenly understanding once you see the reason, which often follows: 「暑いはずだ。エアコンが消えている」 no wonder it's hot — the air conditioner is off.
道理で寒いはずだ。窓が開いていた。
No wonder it's cold — the window was open.
道理で田中さんは料理がうまいはずだ。お母さんがシェフなんだって。
No wonder Tanaka cooks so well — his mother is a chef.
Mixed up with
Building it: a な-adjective needs 「な」: 「親切なはずだ」; a noun needs 「の」: 「日曜日のはずだ」.
Degree of certainty: 「かもしれない」 is 'maybe'; 「だろう」 is a gut-feeling guess; 「はずだ」 is a conclusion backed by objective grounds, the strongest.