The placement determines which word your are modifying. You speak perfect German. In this case, 'perfect' modifies 'German', and is therefore an adjective. You speak German perfectly. Here, 'perfectly' modifies 'speak', a verb, which makes it an adverb. There is no logical reason for this. It is just one of the rules that we go by. Maybe someone can disagree - I know the rule for this placement - I'm just not sure of the reason.

1, He speaks German. a. "speaks" is the verb. b. "German" is the object. Rule: Usually we do NOT put an adverb between the verb and the object. 2. I like broccoli. a. I really like broccoli. b. Really, I like broccoli. c. I like broccoli, really. d. I like really broccoli. = NO! 3. You write English. a. You write English very well. d. You write very well English. = NO! 4. Mona bakes cakes. a. Mona often bakes cakes. b. Mona bakes often cakes. = NO!

If you use "PERFECTLY" (ADVERB) so you are saying "HOW" he speaks. You are telling us "HOW GOOD HE IS SPEAKING GERMAN"

He speaks German perfectly. He speaks perfect German. He speaks A perfect German... is not quite right. But you may have HEARD the very similar structure... "He's done A perfect job on the car (repairing it)." "She produced a perfect answer." (one answer of a range of possible answers). Actually, this is quite hard to explain... so I think I need a cup of coffee. Someone else can help explain. Please!