Learn Hardcore Romanian: Întrebarea este ușor de înțeles.

In the “ușor de + participle” construction, ușor functions adverbially (or as an indeclinable predicate) and never agrees with the subject in gender or number. It remains in its base form regardless of whether the noun is masculine, feminine, singular, or plural.

What is the role of the preposition de before înțeles? Why not use a înțelege?

Romanian uses the pattern adjective + de + past participle to express “easy/difficult to [verb].” Here, de links ușor to the action. You don’t use the full infinitive (a înțelege) in this construction—only the past participle (înțeles) follows de.

What exactly is înțeles in this sentence?

Înțeles is the past (perfect) passive participle of a înțelege (“to understand”). In ușor de înțeles, it conveys “to be understood” and, together with de, completes the idea “easy to understand.”

How would you say the opposite, “hard to understand”?

Simply replace ușor with greu:
Întrebarea este greu de înțeles.

Can I rephrase it using a passive construction with se, like “Întrebarea se înțelege ușor”?