Unit 5 - Section 3: 〜はずだ
〜はずだ (ought to / I expect that)
はずだ is an expectation based on a reason you have. If reality disagrees, you are surprised, not just guessing.
もう着いているはずです。
They should have arrived by now. (given the timetable)
昨日勉強したから、できるはずだ。
I studied yesterday, so I ought to be able to do it.
来るはずの人が来なかった。
The person who was supposed to come did not come.
はずがない is a strong “there is no way”: 彼が忘れるはずがない.