Expectation with わけ, はず, べき, ものだ, and かもしれない

わけ is a noun meaning "reason" (as in "why"). It can be used in many different set expressions. It is not possible to list all of them here but just a few common ones.

At the core わけ(だ) states a reason, explanation, fact, logical restatement, or conclusion. This usage can occasionally be replaced with のだ.

なるほど、そういう可能性もあるわけだ。
I see, so that possibility also exists.

それならいくら探しても見つからないわけだ。
If that’s the case, no matter how hard they search, she won’t be found.

わけがない means “there is no way” and implies that something is not even remotely thinkable by the speaker

そんな馬鹿なことがあるわけがない。
There’s no way such a ridiculous thing would exist.

わけじゃない on the other hand is a roundabout way to negate a situation, circumstance, or logical conclusion.

可能性がないわけじゃない。
It’s not like there’s no chance.

わけにはいかない means that something is not possible according to general knowledge or social expectations.

このチャンスを逃すわけにはいかない。
I cannot possibly let this chance go.

Since わけ is a very strong statement, it is often paired with という in the form of というわけ to soften the impact of the expression.

はず expresses that the speaker expects something to factually be true or come true. It can be similar to the English “should” but it does not cover the entire same range of situations. はず also implies that the expectation is based on reasoning and is not pure conjecture.

こんなはずじゃなかった、こんなはずじゃなかった
It shouldn’t have been like this, it shouldn’t have been like this

さっきまで自分の部屋にいたはずなのに。
But she should’ve been in her room until a moment ago...

その場にいた全員がそう思ったはずだ。
Everybody standing there must have thought that.

べき states that the speaker expects a certain behaviour or state to be upheld, because not doing so would be bad, irresponsible, etc.

この場から立ち去るべきだ。
It would be good to leave this place.

わからないことは聞くべきだ。
You should ask about the things you don’t understand.

There are however some situations where using べき isn't always so heavy, especially in questions when asking for someone's wisdom or advice.

どこに行くべきだと思う？
Where do you think we should go?

When in the past tense like べきだった, it says that an action or state "was" or "would have been" right, usually the latter.

もっと気を付けるべきだった。
We should have been more careful.

べし is an archaic version of べき. Just be aware it exists.

ものだ can also express "should" with the nuance that it's a general "should" that applies to a lot of people, not just a specific person or a group. It is often used to state the nature of “how things are”.

子供は親の言う事を聞くものだ。
Children should listen to what their parents say.

かもしれない is a compound sentence-ender. It attaches the same way as か alone does. It expresses that the speaker thinks the statement is a possibility. Sometimes people attach this to things they know are true for politeness's sake.

まだ寝ぼけているかもしれない。
Maybe she’s still half asleep.

かもしれない is often shortened to just かも in casual conversation.