Neither sentence is grammatical English or something that a native would typically say, so it is pointless to argue over what they might mean. I don't see a reason to correct them differently.
“*This would not have been happened” is a passive construction. But happen is not a transitive verb, so it cannot be used in the passive voice. It is likely that the author of this sentence intended to write “This would not have happened”, i.e. the event mentioned previously would not have taken place.
In addition to the previous defect, “*this would have not been happened” is incorrect because the negation cannot be placed where it is. The negation word not is normally placed after the first auxiliary: “this would not have …”.
Furthermore, the second part of the sentence is in the wrong tense. “This would have happened” is a past conditional. The part of the sentence introduced by if is a past hypothetical. It needs to be in the past tense because the time of the action is in the past; in addition, the hypothetical aspect is expressed by a past tense, which means the verb needs to be in the pluperfect tense: “if you had not been there at that time”.
A correct sentence resembling these two incorrect sentences is
This would not have happened if you had not been there at that time.
I can't think of a correct sentence with a different meaning that could be what the author was trying to express.
1) Monday should be fine.
2) Monday would be fine.
Should in sentence 1 is not the same use of should that we find in examples such as:
- What should I do?
- You should apologise.
- You should eat five portions of fruit and vegetables every day.
The sentences above are asking for advice or giving advice. We can think of advice here like a form of weak obligation. If someone gives us advice, there is some pressure on us to do that thing - but we don't have to do it - it is our decision. This kind of meaning, when we talk about obligation and permission, is called DEONTIC modality.
Sentence 1 is NOT about deontic modality. This type of should is about EPISTEMIC modality. Epistemic modality is about knowledge and belief. Think about the following sentences:
- It might be in box 3.
- It should be in box 3.
- It must be in box 3.
The first example above shows that the speaker has a weak belief that it is in box 3. The second sentence shows that the speaker has a fairly strong belief that it is box 3, but she's not certain. The last example shows that the speaker is certain that it is in box 3.
In the Original Poster's example, (1) indicates that the speaker has a strong conviction, a strong belief, that Monday will be fine. Of course this is the technical meaning of what they are saying. The effect of saying this sentence is probably "Yes, choose Monday".
Would in sentence (2) indicates a logical result of choosing Monday. The sentence is like the last part of a conditional:
- If you chose Monday, Monday would be fine.
Here the speaker is definite about the fact that the result of choosing Monday is it's being fine. They aren't indicating any doubt about it. Again the effect in the conversation is probably "Yes, choose Monday".
Hope this is helpful!
Best Answer
There's no difference between the meanings, but your second example would likely never be used by an educated native speaker.
The problem is that "have been" is the present perfect continuous form of the verb "to be." In other words, the two words belong together to create one compound verb. They should never be split apart.
Therefore, while the meanings of the two examples are the same. #1 is grammatically correct and #2 is not.