Let's say you watched a war-related movie, wherein the bomb was dropping down a city. And then, you storytell it to your interested friend.
Oh man, the movie was so intense! The aircraft dropped the bomb too fast!/ too fastly!
I think I've heard or read the usage of too fast similar to my example, and this is ungrammatical(I think) that even natives say that.
So is this an exception to the grammar rules now?
Best Answer
Many adverbs end in -ly, but not all of them do. Fast can be an adjective, but in this case fast is an adverb, and there's nothing non-standard or ungrammatical about it. Fastly, on the other hand, would be ungrammatical; fast is already an adverb, and there's no need to add -ly.
Dictionaries aren't always right about parts of speech, but in this case they'll give you the right answer. Macmillan Dictionary, for example, defines fast as an adverb which means 'quickly'. As you've noticed yourself, this usage is quite common.
Too fast isn't especially informal, but in formal speech it might be better to say too quickly instead.