Ŝi ridas kvazaŭ la filmo estus komedio.

• Ŝi = she
• ridas = laughs / is laughing
• kvazaŭ = as if / as though
• la filmo = the film / the movie
• estus = would be / were (a special mood form, explained below)
• komedio = a comedy

So the sentence structure is very close to English: She laughs as if the movie were a comedy.

Ŝi is the normal third-person singular feminine pronoun, meaning she.

Si is a special reflexive pronoun in Esperanto. It is only used when the subject refers back to itself in certain third-person situations, such as:

• Ŝi vidas sin. = She sees herself.
• Li lavas sin. = He washes himself.

In your sentence, the subject is simply she, so you use Ŝi, not si.

Ridas has the verb ending -as, which marks the present tense in Esperanto.

• ridas = laughs / is laughing
• ridis = laughed / was laughing
• ridos = will laugh

Esperanto present tense is often broader than English present simple. Depending on context, Ŝi ridas can mean:

• She laughs
• She is laughing

Here it describes what she is doing, so ridas is perfectly natural.

Kvazaŭ means as if or as though.

It introduces a comparison that is not being presented as a real fact, but as an appearance, impression, or imagined situation.

So:

• Ŝi ridas kvazaŭ la filmo estus komedio. means
• She laughs as if the movie were a comedy.

The idea is not necessarily that the movie really is a comedy. It only seems that way from her reaction.

This is one of the most important points in the sentence.

Estus is the conditional form of esti (to be). It often appears after kvazaŭ when the speaker is describing something unreal, doubtful, imagined, or contrary to fact.

• estas = is
• estus = would be / were

So:

• kvazaŭ la filmo estas komedio would sound more like as if the movie is a comedy, treating it more directly as a real possibility or statement.
• kvazaŭ la filmo estus komedio gives the stronger sense of as if the movie were a comedy, which is the more standard and natural pattern in this kind of sentence.

For English speakers, this is similar to English using were after as if in more formal or traditional grammar:

• She laughs as if the movie were a comedy.

Not always.

Grammatically, estus is the conditional form, and a basic translation is would be. But in sentences with kvazaŭ, English often translates it more naturally as were rather than would be.

So in this sentence:

• literal-ish sense: as if the movie would be a comedy
• natural English: as if the movie were a comedy

That means you should understand estus here by function, not only by its most basic word-for-word translation.

• la filmo = the movie
  This refers to a specific movie, presumably one already known from context.

• komedio = a comedy / comedy
  Here komedio is a predicate noun after esti. Esperanto often does not use an article in this kind of phrase unless there is a reason to make it definite.

So:

• la filmo estus komedio = the movie were a comedy

English usually says a comedy, but Esperanto can simply use komedio without an article.

Because it is not a direct object.

The accusative ending -n is used mainly for:

• direct objects
• motion toward something in some expressions

In this sentence:

• Ŝi is the subject
• ridas is the verb
• la filmo estus komedio is the clause after kvazaŭ
• inside that clause, la filmo is the subject and komedio is a predicate noun after estus

Predicate nouns after esti do not take -n.

Compare:

• La filmo estas komedio. = The movie is a comedy.
• not komedion

You may sometimes see kvazaŭ used with the indicative, but estus is the usual and recommended choice when the meaning is clearly as if / as though in an unreal or only apparent sense.

So for a learner, the safest and most standard pattern is:

• kvazaŭ + conditional
• Ŝi ridas kvazaŭ la filmo estus komedio.

Using estas would make the clause feel more factual or less clearly hypothetical, and many speakers would find estus better here.

Yes, Esperanto word order is fairly flexible, but the original order is the most neutral and natural.

Standard:

• Ŝi ridas kvazaŭ la filmo estus komedio.

Possible alternatives for emphasis:

• Kvazaŭ la filmo estus komedio, ŝi ridas.

That said, beginners should usually keep the straightforward order unless there is a special reason to emphasize something.

Yes — probably these three things:

1. -as = present tense

   • ridas = laughs / is laughing

2. kvazaŭ often introduces an unreal comparison

   • as if / as though

3. After kvazaŭ, Esperanto often uses the conditional

   • estus rather than estas

So this sentence is a very good example of a common Esperanto pattern:

• [main clause] + kvazaŭ + [conditional clause]

Example:

• Li parolas kvazaŭ li scius ĉion.
  = He talks as if he knew everything.