Pasa la salsa por el colador antes de servirla.

Because pasa here is the tú imperative of pasar: a command meaning pass/strain it.

• tú pasas = you pass / you strain → statement
• pasa = pass! / strain! → command

So in instructions like recipes, pasa is telling someone what to do.

Spanish uses definite articles much more often than English does.

In a sentence like this, la salsa and el colador sound completely natural because the speaker is referring to specific, understood things in the context: the sauce you are making and the strainer you are using.

English often says strain sauce through a strainer, but Spanish usually prefers pasa la salsa por el colador.

Here, pasar algo por means to pass something through something, especially a tool or process.

So:

• pasar la salsa por el colador = to pass the sauce through the strainer

This is a very common structure in cooking:

• pasar por agua
• pasar por la batidora
• pasar por el colador

The idea is that something goes through a process, container, or tool.

Because por is the normal, idiomatic preposition with this verb in this kind of cooking instruction.

Even though English often says through, Spanish usually says pasar algo por when talking about running food through a strainer, sieve, blender, etc.

• natural: Pasa la salsa por el colador
• more literal but less idiomatic here: a través de

So this is a case where Spanish uses its own standard pattern, not a word-for-word equivalent of English.

A colador is a strainer or sieve.

In cooking, it is something used to separate liquid from solids or to make a sauce smoother.

Depending on the context, English might translate it as:

• strainer
• sieve
• sometimes colander, though that is often a different kind of kitchen tool

In this sentence, strainer is probably the best match.

Because antes de is followed by an infinitive when the subject stays the same or is general.

So:

• antes de servirla = before serving it

This is a very common pattern:

• antes de comer
• antes de salir
• antes de usarlo

If the subject changes, Spanish normally uses antes de que + subjunctive instead:

• Antes de que la sirvas...
• Antes de que se enfríe...

But in your sentence, antes de + infinitive is the normal choice.

Because object pronouns in Spanish can attach to an infinitive.

Here, la refers to la salsa.

So:

• servirla = to serve it
• literally: serve-it

This is completely normal in Spanish. With infinitives, pronouns can be attached directly:

• hacerlo
• comerla
• prepararlo

So servirla is just servir + la.

Yes, you could, especially if the context is already very clear.

• antes de servirla = more explicit, because it repeats the idea of the sauce
• antes de servir = more general, but still understandable in context

Spanish often uses object pronouns to keep the reference clear and natural, so servirla sounds very normal here.

Yes, if the sauce has already been mentioned and you do not need to say la salsa again.

Compare:

• Pasa la salsa por el colador... = mention the noun directly
• Pásala por el colador... = use a pronoun instead, meaning Pass it through the strainer...

Notice the accent mark:

• pasa = no accent
• pásala = accent needed because attaching la changes the stress pattern

Both are correct; they just package the object differently.

Because la salsa is the direct object of servir here.

• servirla = to serve it → la refers to the sauce
• servirle would usually mean to serve him/her/you something

So in this sentence, the thing being served is the sauce, and that takes the direct object pronoun la.

The sentence uses the informal singular command, tú:

• Pasa la salsa por el colador antes de servirla.

Other possibilities are:

• usted: Pase la salsa por el colador antes de servirla.
• vosotros (very common in Spain): Pasad la salsa por el colador antes de servirla.
• ustedes: Pasen la salsa por el colador antes de servirla.

So the version you have is the tú form, which is common in direct instructions.