Except and except for (3)

1. except for is used before nouns/noun phrases

   The garden was empty except for one small bird.

2.except before prepositions and conjunctions
   He's good-looking except when he smiles.
   This room is no use except as a storeroom.

3. except (for) after all, any etc
   After generalising words like all, any, every, no, everything, anybody, nowhere, nobody, whole, we often leave out for. But this does not happen before all, etc.
   Nobody came except(for) John and Mary.
   Except for John and Mary, Nobody came.(not Except John and Mary, Nobody came.) 

4. except(for) + object pronoun, not subject pronoun
   Everybody understood except(for) me.(not …for I)

5. except + verb
   A common structure is do…except +infinitive without to.
      He does nothing except eat all day.
 
   In other cases an -ing form is usually necessary.
      You needn't worry about anything except having a great time.