"The weasels in the library run quickly."
Translation:Mustelae in bibliotheca celeriter currunt.
"Rapide" and "Celeriter" aren't synonyms.
Rapide, according to Gaffiot and Lewish & Short, is mainly used to talk about water, flows, rivers, and it's not only "quick, rapid", but it has an additional meaning of violence. The water flows quickly and violently.
răpĭdē:, hurriedly, hastily, quickly, rapidly:
dilapsus (fluvius). = River (French "fleuve")
Iter confecit (Iter confecit tam festinanter et rapide). Festinanter: hastily.
Cum rapide fertur, sustineri nullo pacto potest = Here, it as a meaning of quickness and strength in the same time, something that is quick and violent and cannot be stopped.
They also note that the adjective "rapidus" is very rare and only poetic.
Etymology for "rapid" in English:
"moving quickly," from French rapide (17c.)
Latin rapidus "hasty, swift, rapid; snatching; fierce, impetuous," from rapere "hurry away, carry off, seize, plunder," from PIE root *rep- "to snatch" (source also of Greek ereptomai "devour,"(...)
It lost its violent connotation (snatching) in old French, so this meaning wasn't passed in English.
So, with the connotation that "rapide" have, it probably depends how the weasel is running, if it bumps, shoves and knocks everything. But it's really not what the sentence says here. If it's simply "quickly", "rapide" is not good, and "celeriter" has to be preferred.
Because Rapide has violent connotations that Celeriter doesn't have.
Gaffiot confirms Lewis & Short, and gives simply "celeriter" = quickly, and "Rapide" = that bring everything with it (French has a very descriptive word "le vent/l'eau emporte tout, I think in English "snatching" is relatively close.), with violence.
So, we understand why it's used mainly for strong water flows, and that it can be used figuratively only when the quickness is like the quickness and fury of fierce waters.
I suspect you are getting too caught up with the listings of that one dictionary. Other dictionaries do not make the same distinction. One must remember, that latin was spoken during a period of around two thousand years, and the meaning of the word might have shifted. Rapido and rapide means quick in italian and french today, without any extra qualities.
Why is the verb conjugated here? On the previous instances the verb would be in the infinitive when together with an adverb. As in 'He usually runs' uses 'ambulare', and conjugates the adverb, instead of the verb. There is no lesson or adverbs, or anything, and no notes on this lesson. I have only studied Latin on this course, where is a good place to study it further or check these grammar problems?