Asegúrate de llevar el pasaporte antes de salir.

Because asegurarse de + infinitive / noun is the usual Spanish structure for to make sure (to do something / of something).

So:

• asegurar = to assure, to insure, to secure
• asegurarse de = to make sure

In this sentence, asegúrate is the tú affirmative command of asegurarse.

So Asegúrate de llevar... means Make sure to take...

If you said asegura, it would usually mean something more like assure or secure, not the intended idea here.

Because this is a reflexive verb: asegurarse.

In affirmative commands, reflexive pronouns are attached to the end of the verb:

• asegúrate = make sure
• levántate = get up
• siéntate = sit down

So:

• base command: asegura
• reflexive pronoun: te
• combined: asegúrate

This is completely normal in Spanish command forms.

The accent mark is needed because when te is attached to the command, the stress pattern changes.

Without the written accent, the word would not be pronounced correctly according to normal Spanish stress rules. The accent keeps the stress where it belongs:

• a-se-GÚ-ra-te

This happens often with affirmative commands plus attached pronouns:

• dime
• explícamelo
• asegúrate

So the accent is there for pronunciation and spelling reasons.

The verb asegurarse normally goes with de before an infinitive or noun.

So the pattern is:

• asegurarse de + infinitive
• asegurarse de + noun

Examples:

• Asegúrate de llamar. = Make sure to call.
• Me aseguré de la hora. = I made sure of the time.

So in Asegúrate de llevar el pasaporte, the de is required by the verb structure.

Here llevar means to take / to carry / to bring with you.

In this context, llevar el pasaporte means to have the passport with you when you go.

Spanish often uses llevar where English might say:

• take
• bring
• carry

So Asegúrate de llevar el pasaporte is a very natural way to say Make sure you take your passport.

Tomar usually means to take in senses like taking medicine, taking a taxi, taking a class, etc., not usually take an object with you in this context.

Spanish often uses the definite article (el, la, los, las) where English uses a possessive like my, your, his—especially when possession is obvious from context.

Here, it is clear whose passport is meant: the person being addressed.

So both are possible:

• Asegúrate de llevar el pasaporte.
• Asegúrate de llevar tu pasaporte.

But el pasaporte sounds very natural because the context already tells us it is your passport.

Spanish does this a lot.

Literally, it means before leaving or before going out.

Breakdown:

• antes = before
• de = of / to introduce the infinitive here
• salir = to leave, to go out

So antes de + infinitive is a very common structure:

• antes de comer = before eating
• antes de dormir = before sleeping
• antes de salir = before leaving / before going out

Because in Spanish, antes needs de when it is followed by an infinitive.

So the correct pattern is:

• antes de + infinitive

Examples:

• antes de estudiar
• antes de entrar
• antes de viajar

So antes de salir is correct, while antes salir is not.

It can mean either, depending on context.

Salir is a flexible verb that can mean:

• to leave
• to go out
• to go out of
• to head out

In this sentence, antes de salir most naturally means:

• before leaving
• before heading out

If you are talking about leaving the house, airport, hotel, etc., salir works very naturally.

It is informal singular, because asegúrate is the command for tú.

So this is what you say to one person you address informally.

Other versions would be:

• Asegúrate de llevar el pasaporte. = informal singular
• Asegúrese de llevar el pasaporte. = formal singular
• Aseguraos de llevar el pasaporte. = informal plural in Spain
• Asegúrense de llevar el pasaporte. = formal plural / common in Latin America

Since you specified Spanish from Spain, tú and vosotros forms are especially relevant.

Yes, absolutely. That would mean Don’t forget to take the passport before leaving.

It is slightly different in tone:

• Asegúrate de llevar el pasaporte = make sure you take the passport
• No olvides llevar el pasaporte = don’t forget to take the passport

Both are natural, but asegúrate emphasizes checking or making sure, while no olvides focuses more directly on not forgetting.

Yes. That is also correct and very natural.

There are two common patterns:

• asegurarse de + infinitive
  Asegúrate de llevar el pasaporte.
• asegurarse de que + clause
  Asegúrate de que llevas el pasaporte.

The meaning is very similar.

A rough difference:

• de llevar focuses on the action: make sure to take it
• de que llevas focuses on the fact/state: make sure you have it with you

Both are good Spanish.