Placement
When an adverb modifies anything besides a verb (like an adjective, another adverb, or noun phrases for example) it is placed in front of what it modifies .Je suis vraiment diabolique.Manner Adverbs
I am truly evil.Il a vécu dans la jungle bresilienne très longtemps.
He lived in the Brazilian jungle for a very long time.
In simple sentences the adverb follows the verb.
Je parle étrangement.A big No-No is placing the adverb before the verb - as is possible in English. Doing this in French is outrageous and horribly wrong. (Okay, I might be exaggerating somewhat)
I speak strangely.
But, in sentences that use the passé composé, usually adverbs immediately follow the auxiliary verb (avoir or être).
J'ai complètement oublié de faire le travail.
I completely forgot to do my homework.