Siento que hayas tenido que vomitar en el autobús; a mí también me dan náuseas a veces.

Because siento que is followed by the subjunctive when you express emotion, regret, or a personal reaction to something.

So:

• Siento que... = I’m sorry that... / I regret that...
• After that, Spanish normally uses the subjunctive:
  • hayas tenido

Has tenido is the indicative, which would not fit after siento que in standard Spanish.

A very useful pattern is:

• Me alegra que + subjunctive
• Siento que + subjunctive
• Me molesta que + subjunctive

So this sentence follows a very common rule.

It is the present perfect subjunctive.

It is made of:

• hayas = subjunctive of haber
• tenido = past participle of tener
• que vomitar = had to vomit

So the structure is:

• hayas tenido que + infinitive

This is used for something completed in the past, often connected to the present speaker reaction.

Here, the speaker is reacting now to something that already happened:

• the person had to vomit on the bus
• the speaker feels sorry about it now

Because tener que + infinitive means to have to do something.

So:

• vomitar = to vomit
• tener que vomitar = to have to vomit

In context, had to vomit suggests it was unavoidable or necessary because of how the person felt.

If you said only Siento que hayas vomitado en el autobús, that would mean I’m sorry that you vomited on the bus. That is also possible, but it is slightly different. It focuses on the act itself, not on the fact that the person was in such a bad state that they had to do it.

Yes, often you could.

• Lo siento = I’m sorry
• Siento que hayas tenido que vomitar... = I’m sorry that you had to vomit...

The version with siento que is more explicit because it directly introduces the thing you feel sorry about.

For example:

• Lo siento mucho.
• Siento que hayas tenido que pasar por eso.

Both are natural, but siento que + subjunctive is especially useful when you want to name the situation clearly.

Because en is the normal preposition for being in/on a means of transport in Spanish.

So:

• en el autobús = on the bus
• en el tren = on the train
• en el avión = on the plane

English often says on the bus, but Spanish uses en here.

Also, autobús is a standard word in Spain. Depending on context, you may also hear bus, but autobús is completely normal and neutral.

A mí también adds emphasis and contrast:

• a mí = to me / as for me
• también = too / also

In this sentence, it means something like me too or I do too.

Why use a mí if me is already there in me dan náuseas?

Because Spanish often uses the stressed form for emphasis or clarity:

• A mí también me dan náuseas.

Without a mí, the sentence would still be correct:

• También me dan náuseas a veces.

But a mí también sounds more personal and sympathetic: that happens to me too.

Because dar náuseas is a very common expression in Spanish.

Literally, it is they give me nausea, but naturally it means:

• I feel nauseous
• I get nauseous

So:

• Me dan náuseas = nausea comes over me / I get nauseous

You can also hear:

• Tengo náuseas = I have nausea / I feel nauseous

Both are possible, but me dan náuseas often emphasizes that the feeling comes on, happens to you, or is triggered by something.

In this sentence, that works well because the speaker means it happens sometimes too.

Because in Spanish, náuseas is normally used in the plural.

So Spanish says:

• tener náuseas
• dar náuseas

not usually singular náusea in everyday usage for this meaning.

Also, there is no article here:

• me dan náuseas
• not usually me dan las náuseas in this context

That is just the standard idiomatic expression.

They are related, but not identical.

• Me dan náuseas = I feel nauseous / I get nausea
• Me mareo = I get dizzy / I get motion sick

If the idea is specifically that the bus makes the speaker feel sick in a motion-sickness way, me mareo is very common in Spain.

For example:

• A mí también me mareo a veces en el autobús would not be correct because marearse needs the same person form:
• Yo también me mareo a veces en el autobús

More natural alternatives might be:

• A mí también me mareo a veces en el autobús. → actually incorrect because of the clash between a mí and me mareo? Let's clarify:
  • Yo también me mareo a veces en el autobús.
  • A mí también me da mareo a veces. is less common.
• A mí también me dan náuseas a veces. is correct as written.

So the original sentence is fine, but marearse would be another common option depending on the exact feeling.

Because they do different jobs.

In a mí también me dan náuseas:

• a mí = the stressed pronoun used for emphasis
• me = the indirect object pronoun required by the verb pattern

This kind of doubling is very normal in Spanish:

• A mí me gusta
• A ella le encanta
• A nosotros nos pasa eso

So even though it may look repetitive in English terms, it is standard Spanish.

Yes, a veces is flexible.

In the sentence:

• a mí también me dan náuseas a veces

it simply means sometimes.

You could also say:

• A veces, a mí también me dan náuseas.
• A mí también a veces me dan náuseas.

All of these are possible. Putting a veces at the end is very natural and keeps the sentence easy to process.

Not especially. It is mostly a punctuation choice.

The semicolon separates two closely related ideas:

• the speaker is sorry about the other person’s experience
• the speaker adds that the same thing happens to them too

You could also write:

• Siento que hayas tenido que vomitar en el autobús. A mí también me dan náuseas a veces.
• Siento que hayas tenido que vomitar en el autobús, a mí también me dan náuseas a veces.

The semicolon is a neat, slightly more careful written choice, but it does not change the grammar.

Yes, it is natural and understandable in Spain.

A few comments:

• autobús is standard in Spain
• vomitar is neutral and direct
• me dan náuseas is natural
• siento que + subjunctive is correct and idiomatic

Depending on tone, native speakers might also say things like:

• Siento que hayas tenido que vomitar en el autobús; a mí también me pasa a veces.
• Qué mal, a mí también me dan náuseas a veces en el autobús.
• Lo siento; yo también me mareo a veces en el autobús.

So the original sentence is good Spanish, and there are just other natural ways to express similar ideas.