I know the generic rules, they end in -e and they describe a verb or another adverb.

I've seen that they must go before the verb that they modify, is true?

Furthermore, if I have an adverb that's modifying another adverb, that's modifying a verb, in which order do the first two adverbs go? (For example, 'he ran very quickly')

And my final question, I've seen many words that end in -ŭ (like ankaŭ), are these considered adverbs? If so, do they go before the verb as well?

Thanks for the help!

Kljunar:I know the generic rules, they end in -e and they describe a verb or another adverb.

There are also words that function grammatically much like adverbs but do not end in -e. They are referred to as "e-vortecaj vortoj" in the PMEG. "Nur" and "ankaux" are such words, and I would even put "tre" in this category, since it is not a true e-word (the whole root is "tre", the e is not an ending that is added to root of "tr" )

I've seen that they must go before the verb that they modify, is true?

No. They just need to be close enough to the verb so it's clear what's being modified.

Furthermore, if I have an adverb that's modifying another adverb, that's modifying a verb, in which order do the first two adverbs go? (For example, 'he ran very quickly' )

In this case, it would sound unusual and poetic to put "very" after "quickly". In other cases, you would use the meaning to guide you.

"Li kuras vere rapide" (He runs really quickly) would have a slightly different meaning than "Li kuras rapide, vere" (Truly, he runs quickly)