The word except means ‘not including’. It can be used as a preposition or a conjunction. When used as a preposition, except is followed by a noun.
I haven’t invited anybody except Peter. (= Peter is the only person I have invited.)
Here the noun Peter acts as the object of the preposition except.
Except can also be used as a conjunction. As a conjunction, except is followed by a clause or an adverbial phrase.
I would like to bail him out, except I don’t have any money.
Except can also be used before a conjunction like that, when or if.
She knows nothing about him except that he is young and handsome. (= She knows nothing about him apart from the fact that he is young and handsome.)
He looks handsome except when he sleeps.
That was a good report except for a few spelling and grammatical mistakes.
Both except and except for can be used after words like all, every, no, anybody, nowhere, whole etc. In other cases, we use except for.
She ate everything on her plate except for the pickles. OR She ate everything on her plate except the pickles. (= She didn’t eat the pickles, but she ate everything else.)
Both except and except for are possible after everything.
I haven’t told anybody except / except for Mary. (Both except and except for are possible after anybody.)
Except for Mary, I haven’t invited anybody. (NOT Except Mary, I haven’t invited anybody.)