はずだ and かもしれない: expectation and possibility
Compare an expectation based on known facts with an open possibility. Look for evidence instead of memorizing probability percentages.
Build an expectation from what you know
予定では、電車は10時に着くはずです。 uses the scheduled time as grounds for expecting the train to arrive at ten. It does not claim that the arrival was observed.
Connect nouns with の and na-adjectives with な: 学生のはず and 静かなはず. Verbs and i-adjectives use plain forms, as in 来るはず and 高いはず.
Leave a possibility open
今日は臨時休館かもしれません。 raises the possibility that the building is temporarily closed today. It does not present that possibility as confirmed.
Omit present affirmative だ with nouns and na-adjectives: 休みかもしれない and 便利かもしれない. かもしれません is the polite form.
Check the grounds, not a numerical certainty level
はずだ connects evidence to an expectation; かもしれない leaves a possibility open. Neither should be learned as a fixed probability percentage.
開いているはずでしたが、休みでした。 shows that an expectation can differ from reality. Mark the expected situation separately from the observed result.
Quick check: Is もうすぐ来るはずです natural when a timetable gives you grounds to expect a bus soon?
Show the answer and why
Yes. The timetable supports the expectation. The sentence still does not confirm that the bus actually arrived soon afterward.