L’attaccapanni del corridoio è pieno di cappotti.

Because attaccapanni begins with a vowel sound, the masculine singular article il becomes l’ before it.

So:

• il libro
• l’attaccapanni

This is called elision. The apostrophe shows that the vowel in il has been dropped.

Good question, because it really looks plural.

Attaccapanni is one of those Italian nouns that are invariable, meaning the singular and plural have the same form. In this sentence, it is singular.

You can tell it is singular because of the words around it:

• l’attaccapanni = the coat rack
• è = is
• pieno = full

If it were plural, you would have something like:

• gli attaccapanni del corridoio sono pieni di cappotti
  = the coat racks in the hallway are full of coats

Because the subject is l’attaccapanni, which is singular here.

Even though attaccapanni looks plural, the sentence treats it as one coat rack, so the verb must also be singular:

• l’attaccapanni è = the coat rack is

Not:

• l’attaccapanni sono

Del is a contraction of di + il:

• di = of
• il = the
• del = of the

So del corridoio literally means of the hallway/corridor.

In natural English, this is often translated less literally as:

• the coat rack in the hallway
• the hallway coat rack

In Italian, del corridoio identifies which coat rack we mean: the one associated with the hallway.

Both can be possible, but they are not exactly the same.

• l’attaccapanni del corridoio = the hallway’s coat rack / the coat rack belonging to or associated with the hallway
• l’attaccapanni nel corridoio = the coat rack located in the hallway

So del corridoio is more about identifying the coat rack, while nel corridoio is more about location.

In many contexts, English uses in the hallway, where Italian may prefer del corridoio.

Because pieno must agree with l’attaccapanni, not with cappotti.

Here:

• attaccapanni = masculine singular
• so the adjective must be pieno = masculine singular

Agreement works like this:

• masculine singular: pieno
• feminine singular: piena
• masculine plural: pieni
• feminine plural: piene

Examples:

• L’armadio è pieno di cappotti.
• La stanza è piena di gente.
• Gli armadi sono pieni di vestiti.

Because pieno di is the normal Italian pattern for full of.

So:

• pieno di cappotti = full of coats
• piena di libri = full of books
• pieni di gente = full of people

This is just the standard construction:

• essere pieno di + noun

Because cappotti here means coats in a general, indefinite sense, not specific coats already identified.

So:

• pieno di cappotti = full of coats

This is very natural in Italian.

If you wanted to refer to specific coats, Italian could use an article, but in this sentence that is not the idea. The sentence just means the coat rack is covered or filled with coats in general.

This is a very important spelling point.

• è with an accent = is
• e without an accent = and

So in this sentence:

• è pieno = is full

Not:

• e pieno

The accent matters because these are two different words.

Yes, it is understandable and natural.

A native speaker might also say appendiabiti in some contexts, depending on the kind of coat rack or hanger stand meant, but attaccapanni is perfectly normal.

So the sentence works well as it is:

• L’attaccapanni del corridoio è pieno di cappotti.