"The man dances slowly."
Translation:La viro dancas malrapide.
All "derived" adverbs do - but there are some others which do not.
It is more correct to say that if a word in Esperanto ends in 'e', you know it is an adverb.
BTW, it helps to think of the ending of "dancas" as "-as", not "-s".
The form "danca" has an ending "-a" on it, which is why it is not obvious that the ending of "dancas" is "-as".
When we get to other tenses (past tense, future tense, etc), you will see why this is a helpful way to think of it.
Although even now, with the lessons you've encountered so far (although perhaps it's not really explained well this early in the course?), you could also have "danco" = "a dance".