Learn German: Die schwere Truhe blieb viele Jahre ungeöffnet. - The heavy chest remained unopened for many years.

Using bleiben (here in the simple past as blieb) with an un- prefixed past participle emphasizes a continuous state of non-occurrence. Wurde nicht geöffnet (was not opened) is a passive action, simply stating the event didn't happen. Blieb ungeöffnet focuses on the state of the chest persisting over time.

How is ungeöffnet formed? Can I put un- in front of any past participle?

It is formed by adding un- to the past participle geöffnet (opened). You can do this with many verbs to describe a state that hasn't been achieved, especially transitive verbs (verbs that take a direct object). It works beautifully with verbs like entdecken (unentdeckt) or lösen (ungelöst), but you generally cannot use it with intransitive verbs like schlafen.

Why is there no preposition like für before viele Jahre?

In German, time durations are often expressed directly using the accusative case, known as the accusative of duration. While you might sometimes hear für viele Jahre in casual speech, dropping the für is the standard, traditional, and more elegant way to say 'for many years'.

What is the difference between a Truhe and a Kiste?