Si Laura no llega al seminario, estará haciendo el transbordo todavía.

Because si without an accent means if.

With an accent, sí usually means yes or sometimes himself / herself / itself after a preposition.

So here:

• Si Laura no llega... = If Laura doesn’t arrive...
• Sí Laura no llega... would be incorrect in this sentence.

This is a very common Spanish pattern for real or possible future conditions:

• si + present indicative
• main clause in future, present, or another suitable tense

So Spanish says:

• Si Laura no llega al seminario, estará...

not:

• *Si Laura no llegará...

In English, we do something similar:

• If Laura doesn’t arrive, she’ll still be...

We do not normally say:

• *If Laura won’t arrive, she’ll...

So the Spanish structure is very natural here.

It is an open / real conditional: a condition that is possible and realistic.

Structure:

• Si Laura no llega al seminario = the condition
• estará haciendo el transbordo todavía = the result or conclusion

This type of sentence is used for things that may really happen, not for hypothetical or impossible situations.

Compare:

• Si Laura no llega..., estará... = real possibility
• Si Laura no llegara / llegase..., estaría... = more hypothetical

Because llegar normally uses the preposition a before a destination:

• llegar a casa
• llegar al hotel
• llegar a la estación

Here, a + el contracts to al:

• a + el seminario = al seminario

So llega al seminario means arrives at the seminar.

It is the future of estar + gerund:

• estar → estará
• hacer → haciendo

Together, this forms the future progressive / future continuous:

• estará haciendo = will be doing

In this sentence, it shows that the action will be in progress at the relevant moment. So the idea is not just that she will do the transfer, but that she will still be in the middle of it.

It can do both, depending on context.

In Spanish, the future tense can sometimes express probability or inference:

• Estará en casa = He/She is probably at home
• Estará haciendo el transbordo = She is probably making the transfer

In this sentence, both ideas are close together:

• it can sound like a future consequence
• but it can also sound like a logical deduction: If Laura hasn’t arrived, she must still be making the transfer

So a native speaker may hear a slight nuance of inference, not just plain future time.

Transbordo is a transport word. In Spain, it usually means a transfer or change from one train, metro, bus, etc., to another.

So hacer el transbordo means something like:

• to make the transfer
• to change trains / lines / buses
• to be in the middle of changing connections

In Spain, this is a very normal expression in public transport contexts.

Because Spanish often uses hacer in fixed or semi-fixed expressions where English may use a more specific verb.

Here, hacer el transbordo is idiomatic. It is similar to other Spanish combinations where hacer works as a kind of general action verb.

You may also hear:

• hacer transbordo
• hacer un transbordo

The version with el can sound like a specific transfer already understood from the situation.

Todavía here means still.

It tells you that, at the relevant moment, the transfer is not finished yet. So the sentence is not just saying Laura will be transferring, but that she will still be transferring.

Compare:

• estará haciendo el transbordo = she will be making the transfer
• estará haciendo el transbordo todavía = she will still be making the transfer

So todavía adds the idea of continuation.

Yes. Its position is fairly flexible.

For example:

• Si Laura no llega al seminario, todavía estará haciendo el transbordo.
• Si Laura no llega al seminario, estará todavía haciendo el transbordo.
• Si Laura no llega al seminario, estará haciendo todavía el transbordo.

All of these are possible, although some may sound more natural than others depending on rhythm and emphasis. The original sentence is perfectly fine.

Because the si clause comes first.

In Spanish, when a subordinate clause like Si Laura no llega al seminario appears before the main clause, it is normal to separate it with a comma:

• Si Laura no llega al seminario, estará haciendo el transbordo todavía.

If the order is reversed, the comma is usually not used:

• Laura estará haciendo el transbordo todavía si no llega al seminario.

That would shift the focus slightly.

• Si Laura no llega al seminario... = If Laura doesn’t arrive / doesn’t make it to the seminar
• Si Laura no ha llegado al seminario... = If Laura hasn’t arrived at the seminar

The version with no ha llegado is more clearly linked to up to now. It sounds more like checking the present result.

The version with no llega is broader and often more natural in this kind of conditional statement about what happens if she does not arrive by the expected time.

Yes. Some possible alternatives are:

• Si Laura no llega al seminario, todavía estará haciendo transbordo.
• Si Laura no llega al seminario, seguirá haciendo el transbordo.
• Si Laura no llega al seminario, es que todavía está haciendo el transbordo.

These are not exactly identical in nuance, but they are all natural. The original sentence is completely correct and sounds like a normal way to express that she is still in the middle of the transfer.