La fojero estas malplena dum la spektaklo.

In Esperanto, nouns normally end in -o.

So:

• fojero = a noun
• spektaklo = a noun

This is one of the most regular parts of Esperanto grammar: if you see -o, you can usually expect a noun.

La is the Esperanto word for the.

So:

• la fojero = the foyer
• la spektaklo = the performance/show

Esperanto has only one definite article, la, and it does not change for gender, number, or case.

Also, Esperanto does not have a separate word for a/an. A bare noun can mean a or an depending on context.

Estas is the present-tense form of esti, meaning to be.

So estas means is / am / are, depending on the subject.

Examples:

• mi estas = I am
• vi estas = you are
• la fojero estas = the foyer is

Esperanto verbs do not change form based on the subject, so estas stays the same with every person.

Malplena is built from:

• plena = full
• mal- = the opposite

So:

• plena = full
• malplena = empty

The prefix mal- is very common in Esperanto and is often used to make opposites:

• bona = good → malbona = bad
• granda = big → malgranda = small
• fermi = to close → malfermi = to open

Because malplena is an adjective, and adjectives in Esperanto end in -a.

It describes la fojero, so it has adjective form:

• fojero = noun
• malplena = adjective describing that noun

In Esperanto, adjectives also agree with the noun in number and case. Here the noun is singular and not accusative, so the adjective is simply malplena.

If it were plural, it would be:

• La fojeroj estas malplenaj. = The foyers are empty.

Dum means during when it comes before a noun phrase, as it does here:

• dum la spektaklo = during the performance

It can also mean while when followed by a clause:

• Dum la spektaklo daŭras, la fojero estas malplena.
• While the performance is going on, the foyer is empty.

So dum can work a bit like either during or while, depending on what follows it.

Because dum is a preposition, and after a preposition Esperanto normally uses the regular noun form, not the accusative.

So:

• dum la spektaklo is correct

The -n ending is mainly used for:

1. direct objects
2. motion toward something in certain expressions

Neither of those applies here, so spektaklo stays without -n.

No. This is the most straightforward word order:

• La fojero = subject
• estas = verb
• malplena = description
• dum la spektaklo = time expression

But Esperanto word order is fairly flexible. You could also say:

• Dum la spektaklo, la fojero estas malplena.

That puts more emphasis on the time phrase during the performance.

Yes, that is possible.

Esperanto sometimes allows an adjective-like idea to be expressed directly as a verb:

• malplenas = is empty

So both of these can work:

• La fojero estas malplena dum la spektaklo.
• La fojero malplenas dum la spektaklo.

The version with estas malplena is often easier for learners to recognize, because it matches the familiar to be + adjective pattern.