"You speak very slowly."
得 (de) is a structural particle which tells us the condition or quality of the way the person speaks (or does something else, as 得 is most often associated with verbs).
"You speak<--得 very slowly."
The tips and notes section on this page is also helpful: https://www.duolingo.com/skill/zs/Telephone
So you can't say "You + speak + very(/is) + slow(/ly)", you have to add 得 between the verb and the adverb? Is there a more general rule about particles like this, or is this a peculiarity of verbs or verb+adverb combos? It is quite frustrating that the only hints for 得 are "need" and "get" - that was really confusing when trying to interpret the sentence.
Contrary to popular belief: yes, there is grammar in Chinese! Take a look at chinesegrammarwiki webpage for a compendium of grammar rules! Remember that most adjectives works in a similar way to verbs in Chinese, that's illustrated by using 很 (an adverb!) so often with adjectives, so in a sense, putting both of them together kinda creates a competition of rules in the sentence, therefore a grammatical rule exists to clear thing out! ;) In this case, you're looking for "the three 'de'" particles!
'Pleco' (excellent educational Chinese language app!) tells me this 'de' is a "structural particle: used after a verb (or adjective as main verb), linking it to following phrase indicating effect, degree, possibility etc".
I think I only kind of understand what that means and how to then use 'de' myself but I hope it helps you!^^