Breakdown of Hay que leer el tratado del cual hablaron.
de
from / of
leer
to read
hablar
to speak / to talk
hay que
(with que) one has to / must
Questions & Answers about Hay que leer el tratado del cual hablaron.
Why does it say del cual instead of just el cual?
The relative pronoun el cual means "which" and refers back to el tratado (the treaty). However, because they spoke "about" the treaty, we need the preposition de. When de is followed by el, they combine into the mandatory contraction del, creating del cual (about which).
Can I just put the preposition at the end, like "el cual hablaron de"?
No, in Spanish you can never end a sentence or clause with a preposition. If a verb like hablar needs a preposition like de, that preposition must be placed directly before the relative pronoun. This is why it translates strictly as "about which they spoke".
Why is de used here if the English translation says "about"?
While de usually translates to "of" or "from", it is the standard preposition used with verbs of communication to mean "about". For example, hablar de means "to speak about" or "to talk about".
Why does the sentence use Hay que for "We must" instead of Tenemos que?