Llega con tiempo a la estación, no sea que cambien el andén a última hora.

Yes. Llega here is the affirmative tú imperative of llegar.

For many -ar verbs, the affirmative tú command looks the same as the él/ella/usted present form:

• llegar → llega
• hablar → habla
• tomar → toma

So Llega con tiempo means Arrive early / Get there with time to spare.

A useful comparison:

• Llega = affirmative command to tú
• No llegues = negative command to tú

So the first part is giving advice or an instruction.

No, they are close but not the same.

• con tiempo = with plenty of time, in good time, early enough
• a tiempo = on time, in time

In this sentence, Llega con tiempo suggests arriving earlier than strictly necessary, so that you have a safety margin.

Compare:

• Llega con tiempo a la estación.
  Arrive at the station with time to spare.

• Llegó a tiempo al tren.
  He arrived in time for the train.

A native English speaker often wants to translate both as on time, but con tiempo is more like give yourself extra time.

No sea que is a set expression meaning something like:

• in case
• lest
• for fear that

It introduces a possible unwanted situation.

So:

• no sea que cambien el andén a última hora

means something like:

• in case they change the platform at the last minute
• lest they change the platform at the last minute

In everyday English, lest sounds formal, but it helps show the structure clearly.

No. No sea que is not the negative command don’t be.

It is a fixed expression built around sea, the present subjunctive of ser, in an impersonal structure. You can think of it loosely as:

• let it not be that...
• may it not happen that...

So no sea que does not refer directly to tú.

That is why it is sea, not seas.

This is one of those places where translating word by word is not very helpful. It works better to learn no sea que as a chunk meaning in case / lest.

Because no sea que triggers the subjunctive.

The clause after no sea que expresses a possibility that the speaker wants to avoid or is worried about, not a simple statement of fact. That is exactly the kind of context where Spanish often uses the subjunctive.

So:

• cambien = present subjunctive of cambiar
• not cambian = present indicative

Compare the idea:

• Cambian el andén.
  They are changing / they change the platform.
  This sounds like a factual statement.

• No sea que cambien el andén.
  In case they change the platform.
  This is a feared or possible event, so subjunctive is used.

The subject is not stated explicitly, but it is understood as an indefinite they:

• the railway staff
• the station
• the train company
• the people in charge

Spanish often leaves subjects out when they are obvious from the context or when the exact identity is unimportant.

So cambien el andén means they change the platform, but the sentence does not need to say exactly who they are.

In Spain, andén usually means platform at a train or metro station.

So here:

• cambien el andén = they change the platform

This is a very common travel word in Spain.

A couple of useful related words:

• la estación = station
• el tren = train
• el andén = platform
• la vía = track

So if you are travelling in Spain, andén is an important word to know.

Because estación is a noun, and in Spanish you normally use an article where English often does too.

Here, a la estación simply means to the station.

Spanish uses the article naturally with this kind of destination:

• Voy a la estación.
• Llega a la estación.
• Nos vemos en la estación.

Even if English might sometimes say just go to station in a very compressed style, normal Spanish needs la here.

Literally, yes, but idiomatically it means:

• at the last minute
• at the last moment
• right before it happens

So:

• cambien el andén a última hora

means that the platform might be changed very shortly before departure, which is a common travel situation.

It does not necessarily mean a full hour before. It is an idiomatic expression.

No sea que is correct and natural, but in everyday speech many speakers also say:

• no vaya a ser que

So you may hear:

• Llega con tiempo a la estación, no vaya a ser que cambien el andén a última hora.

That version is very common in conversation and has almost the same meaning.

Very roughly:

• no sea que = a bit more compact, sometimes slightly more formal or literary
• no vaya a ser que = very common in everyday spoken Spanish

Both are good to understand.

Yes. The sentence is natural, but Spanish offers several similar ways to express the same advice:

• Llega con tiempo a la estación.
• Vete con tiempo a la estación.
• Ve con tiempo a la estación.
• Llega pronto a la estación.

There are small differences:

• llega con tiempo focuses on arriving with extra time
• ve / vete con tiempo focuses on setting off early enough
• llega pronto means arrive early, but it does not always suggest the same idea of a safety margin as clearly as con tiempo

So the original sentence is a very natural choice if the point is leave yourself enough buffer time.