It is not allowed. You ..............................park your car here.
It isn't necessary, but it is a good idea.
Frustrated means...
Do not do this. It is prohibited (not allowed).
A branch might fall out of that tree, You .......................... park your car here.
Pessimistic means...
You need to do this. It is compulsory.
It is a bad idea to do this.
Optimistic means...
...positive state of mind and thinking that things will go well.
You must do it.
shouldn't
...negative state of mind and thinking that things will go badly.
You shouldn't do this. (You should not do this.)
You must not do it.
You should do it.
mustn't
...feeling annoyed that you can't do something or that something is not as you think it should be.