Ni spektas la antaŭfilmon.

Because -as is the present-tense verb ending in Esperanto.

• spekti = to watch
• spektas = watch / are watching

So Ni spektas means we watch or we are watching. Esperanto does not usually need a separate form like English are watching for the present progressive.

Ni means we.

It is the first-person plural pronoun:

• mi = I
• vi = you
• li / ŝi / ĝi = he / she / it
• ni = we
• ili = they

So the sentence begins with the subject: Ni = we.

La is the definite article, meaning the.

Esperanto has only one definite article:

• la libro = the book
• la antaŭfilmo = the trailer

It does not change for gender, number, or case. So la always stays la.

The -n marks the direct object in Esperanto.

In this sentence:

• Ni = the subject, the ones doing the action
• spektas = the action
• la antaŭfilmon = the thing being watched

So la antaŭfilmo becomes la antaŭfilmon because it is the direct object.

This is one of the most important grammar points in Esperanto:

• noun base ending: -o
• direct object ending: -on

So:

• antaŭfilmo = a trailer
• antaŭfilmon = a trailer, as the thing being watched

It is a compound word:

• antaŭ = before
• filmo = film, movie

So antaŭfilmo literally means something like before-film, which is the Esperanto word for a trailer or preview.

Compound words are very common in Esperanto, and they are often easy to understand once you know the parts.

Yes, Esperanto word order is fairly flexible, but this is the most neutral and natural order:

subject + verb + object

So:

• Ni = subject
• spektas = verb
• la antaŭfilmon = object

Because the object has the -n ending, Esperanto can sometimes change the order without losing the meaning. But for learners, Ni spektas la antaŭfilmon is the clearest and most standard version.

It can mean either, depending on context.

Esperanto present tense -as covers both ideas:

• Ni spektas la antaŭfilmon can mean We watch the trailer
• or We are watching the trailer

Usually the situation makes it clear which meaning is intended.

A helpful rough guide is:

an-TAUW-feel-mon

A few details:

• aŭ sounds somewhat like ow in cow
• fi sounds like fee
• stress in Esperanto is always on the second-to-last syllable

In antaŭfilmon, the stress falls on FIL: an-taŭ-FIL-mon

Because la never changes.

Even when a noun takes -n, the article stays the same:

• la filmo = the movie
• la filmon = the movie, as a direct object

So in this sentence, only antaŭfilmo changes to antaŭfilmon.

It is fairly general and means to watch or to view.

You can use it for many things, for example:

• spekti filmon = to watch a movie
• spekti televidon = to watch television
• spekti matĉon = to watch a match/game

So in this sentence, spektas is a normal and natural choice for watching a trailer.