Llega con tiempo a la terminal, por si cambian la puerta de embarque.

Here llega is the affirmative tú imperative of llegar, so it means arrive or get there as a command.

• llegas = you arrive
• llega = arrive! (informal singular command)

So the sentence is speaking to one person in an informal way.

Spanish often leaves out subject pronouns when they are clear from the verb form.

So instead of saying Tú llega..., Spanish simply says Llega...

In fact, Tú llega would sound unusual here. The verb already tells you who the command is for.

Con tiempo is a very common expression meaning with enough time, in good time, or early enough.

It does not mean literally with time in the English sense. In this sentence, it means something like:

• arrive well in advance
• arrive with time to spare
• arrive early enough

A related phrase you may also hear is con antelación, which is a bit more formal and also means in advance.

Because terminal here is treated as a feminine noun: la terminal.

So:

• a + la = a la
• a + el = al

Since it is la terminal, the sentence correctly says a la terminal.

In airport Spanish, la terminal means the terminal building/area.

Por si means in case.

It introduces a possible situation that someone wants to be prepared for.

So:

• Llega con tiempo... = arrive early
• por si cambian la puerta de embarque = in case they change the boarding gate

A useful thing to remember is that por si is different from para que:

• por si = in case
• para que = so that

Because after por si, Spanish normally uses the indicative, not the subjunctive.

So:

• por si cambian... = in case they change...
• not por si cambien...

This is a very common point for English speakers, because in case can feel hypothetical in English, but Spanish still usually uses the indicative after por si.

Examples:

• Lleva un paraguas por si llueve. = Take an umbrella in case it rains.
• Te llamo por si hay noticias. = I’ll call you in case there is news.

In cambian la puerta de embarque, the verb is third person plural: they change.

Spanish often uses an unstated they when the exact people are not important or are obvious from context. Here it means something like:

• the airline staff
• the airport staff
• the system/people in charge

So cambian does not need an explicit subject. It is a natural way to say in case they change the gate.

Because the idea is that someone changes the gate assignment.

• cambian la puerta de embarque = they change the boarding gate
• cambia la puerta de embarque would suggest the gate changes, which is less natural in this context

In airports, the gate does not really change by itself; the airport or airline changes which gate your flight uses.

So the plural cambian sounds more idiomatic.

Literally, puerta de embarque means boarding gate.

Breakdown:

• puerta = door/gate
• embarque = boarding

At an airport, la puerta de embarque is the gate where passengers board the plane.

In everyday English, we usually just say gate, but Spanish commonly says the full expression puerta de embarque.

Yes, this version is informal singular, addressed to tú.

Other versions:

• Informal singular: Llega con tiempo...
• Formal singular (usted): Llegue con tiempo...
• Informal plural in Spain (vosotros): Llegad con tiempo...
• Formal plural / ustedes: Lleguen con tiempo...

So if you were speaking politely to one person in Spain, you would say:

Llegue con tiempo a la terminal, por si cambian la puerta de embarque.

Yes, you could say Llega temprano, and it would be understandable, but the nuance is slightly different.

• temprano = early
• con tiempo = with enough time / leaving a safe margin

Llega con tiempo sounds especially natural when the idea is allow extra time for possible problems or changes. That fits this airport context very well.

So con tiempo is a particularly good choice here because the second part of the sentence gives a reason: por si cambian la puerta de embarque.