Si pierdo el metro, voy a llegar tarde a clase.

Because after si for a real, possible future condition in Spanish, you normally use the present tense, not the future.

So Spanish says:

• Si pierdo el metro, voy a llegar tarde.

Literally, that is something like:

• If I miss the metro, I’m going to arrive late.

English often uses present after if too: If I miss the metro..., not If I will miss the metro...

So this structure is very normal:

• Si + present, then a future idea in the main clause.

Examples:

• Si llueve, no salgo.
• Si tengo tiempo, te llamo.
• Si pierdo el metro, llegaré tarde.

In Spanish, the present tense is often used after si to talk about a future possibility.

So si pierdo el metro does not mean only if I am missing the metro right now. In this context, it means if I end up missing the metro / if I miss the metro.

This is just how Spanish expresses this kind of condition:

• Si estudio, aprobaré. = If I study, I’ll pass.
• Si llegas tarde, empezamos sin ti. = If you arrive late, we’ll start without you.

This is a real or likely condition: a normal if sentence about a possible situation.

The pattern is:

• Si + present tense, + future / ir a + infinitive / command / present

In your sentence:

• Si pierdo el metro = condition
• voy a llegar tarde a clase = result

Other examples:

• Si pierdo el autobús, cogeré un taxi.
• Si hace frío, me pongo un abrigo.
• Si tienes hambre, come algo.

So this is one of the most common and useful Spanish condition patterns.

Spanish often uses the definite article where English might not.

Here, el metro means the metro / the subway, but in natural English we often just say miss the metro or miss the subway anyway, so this matches fairly well.

Spanish transport nouns commonly take the article:

• coger el autobús
• perder el tren
• tomar el metro

So pierdo el metro is the normal way to say I miss the metro.

No. Here el metro means the metro / subway system.

Spanish metro can mean different things depending on context:

• el metro = the metro, subway
• un metro = a meter, the unit of length

In this sentence, because of pierdo el metro, it clearly means transport.

Both are correct.

• voy a llegar tarde = I’m going to arrive late
• llegaré tarde = I will arrive late

In everyday spoken Spanish, ir a + infinitive is extremely common, especially for near-future or very likely situations.

So:

• Si pierdo el metro, voy a llegar tarde a clase.
• Si pierdo el metro, llegaré tarde a clase.

Both sound natural. The version with voy a llegar can feel a bit more immediate or conversational.

Usually the difference is small.

Very generally:

• voy a llegar tarde often sounds a bit more conversational and immediate
• llegaré tarde can sound a bit more neutral, straightforward, or slightly more formal

In many everyday contexts, native speakers could use either one.

So for a learner, the important thing is:

• both are grammatically correct
• both mean basically the same thing here

Because in Spanish, being late is usually expressed with llegar tarde when talking about arriving late somewhere.

• llegar tarde = to arrive late

Examples:

• Llego tarde al trabajo.
• Siempre llegas tarde.
• Si pierdo el metro, voy a llegar tarde.

Spanish can also say:

• Es tarde = It’s late (talking about the time)
• Se me hace tarde = It’s getting late for me / I’m running late

But for arriving late, llegar tarde is the normal expression.

Because ir a clase is a common expression meaning to go to class or to class in a general sense.

When you are talking about the activity or destination in a general way, Spanish often drops the article:

• Voy a clase.
• Estoy en clase.
• Después de clase...

But if you mean a specific class, the article may appear:

• Voy a la clase de español de las seis.
• Llegué tarde a la clase de historia.

So in your sentence, a clase means to class in a general everyday sense.

Yes, you can, and it can sound very natural in spoken Spanish.

Spanish often uses the present tense to talk about a future result when the meaning is clear:

• Si pierdo el metro, llego tarde a clase.

This has a very immediate, matter-of-fact feeling:

• If I miss the metro, I arrive late / I’m late to class.

Still, for learners, these are all good options:

• Si pierdo el metro, voy a llegar tarde a clase.
• Si pierdo el metro, llegaré tarde a clase.
• Si pierdo el metro, llego tarde a clase.

Because with si for real, possible conditions, Spanish normally uses the indicative, not the subjunctive.

So:

• Si pierdo el metro... is correct.

The subjunctive is not normally used directly after si in this kind of sentence.

Compare:

• Si tengo tiempo, voy. = real/possible condition
• Si tuviera tiempo, iría. = hypothetical/unreal condition

So the key contrast is not indicative vs subjunctive here, but rather:

• present indicative for real possibilities
• imperfect subjunctive for hypothetical situations

Then Spanish would use a different pattern:

• Si perdiera el metro, llegaría tarde a clase.

This is the standard hypothetical condition:

• Si + imperfect subjunctive
• conditional

So compare:

• Si pierdo el metro, voy a llegar tarde a clase.
  Real possibility: If I miss the metro, I’m going to arrive late to class.

• Si perdiera el metro, llegaría tarde a clase.
  More hypothetical: If I missed the metro, I would arrive late to class.

Yes. Perder is the normal verb for to miss in the sense of fail to catch transport.

Examples:

• perder el tren
• perder el autobús
• perder el avión
• perder el metro

Be careful: miss in English has different meanings, and Spanish uses different verbs depending on the meaning.

For example:

• I miss the train → Pierdo el tren
• I miss my friends → Echo de menos a mis amigos

Yes, depending on the transport.

For example:

• Si pierdo el autobús, voy a llegar tarde a clase.
• Si pierdo el tren, voy a llegar tarde a clase.
• Si pierdo el tranvía, voy a llegar tarde a clase.

In Spain, metro is the normal word for the underground/subway system in cities that have one.

It breaks down like this:

• Si = if
• pierdo = I miss / I lose
• el metro = the metro
• voy a llegar = I am going to arrive
• tarde = late
• a clase = to class

So the structure is:

• Si + present tense → condition
• ir a + infinitive → future result

This is a very useful pattern to copy:

• Si salgo tarde de casa, voy a perder el autobús.
• Si no estudio, voy a suspender.
• Si hace calor, voy a abrir la ventana.

Yes, it sounds natural and completely normal in Spain.

A speaker in Spain would easily say:

• Si pierdo el metro, voy a llegar tarde a clase.

Depending on style and context, they might also say:

• Si pierdo el metro, llegaré tarde a clase.
• Como pierda el metro, llego tarde a clase. (more colloquial/emphatic in some contexts)

But the original sentence is standard, clear, and natural.