This is the 29th most frequent Greek word.
δεν
"Δεν" means "not" or "does not" in English.
In this sentence, "δεν" is used as a negation particle preceding the verb "θέλω" (want) to indicate that the speaker does not desire the action referenced.
Δεν θέλω να πάω στο πάρτι.
I do not want to go to the party.
Here, "δεν" acts as a negation to the verb "μπορώ" (can), implying inability to perform the action of finding the keys.
Δεν μπορώ να βρω τα κλειδιά μου.
I cannot find my keys.
In this context, "δεν" negates the action of the verb "έφτασε" (arrived), indicating the state of the subject, the bus, as not yet having completed the action of arriving.