Hi,
She better be awake.
She better sing nicely.
Are those examples of the imperativ or subjunctive? They are really examples of giving advice, similar to 'you should. . , you ought to . . .'. The full structure is 'had better', but in informal speech the 'had' is often omitted. She had better be wake. She'd better be awake. She better be awake.
She had better be awake. She had better sing nicely.
What about following:
It's better when she is awake. Factual statement. 'This has happened'. She has woken up in the past and will wake up in the future, and on such occasions it has been and will be better.
It's better when she sings nicely. Likewise
In what ways is it different to:
It would be better if she were awake. Hypothetical. She may never wake up, and thus it may never be better.
It were better if she would be awake. I wouldn't rush to say this is wrong, but it's archaic tone sounds loke very contorted English to a modern ear.
It were better if she were awake. Very hypothetical, unreal situation.
It would be better if she sang nicely. Say it this way.
Best wishes, Clive