El taxista no dejó de tocar el claxon, y por eso mi hermana se puso nerviosa.

Because dejar de + infinitive means to stop doing something.

• dejó de tocar = he stopped honking
• no dejó de tocar = he did not stop honking

So in natural English, that often becomes he kept honking or he wouldn’t stop honking.

It is a very common structure meaning to stop + -ing.

Examples:

• Dejó de fumar = He/She stopped smoking
• Dejé de leer = I stopped reading

In this sentence:

• no dejó de tocar el claxon = he didn’t stop honking the horn

So the negative changes the idea from stopped to didn’t stop.

Dejó is the preterite, which is often used to tell events in a story as complete actions.

Here the sentence presents a sequence:

1. The taxi driver kept honking.
2. As a result, the sister became nervous.

Using dejó helps present that first part as a whole event in the narrative.

If you said no dejaba de tocar el claxon, it would sound more like background description:

• The taxi driver was going on honking...

Both can work in some contexts, but no dejó de tocar fits a straightforward past event very well.

Yes. That is very natural too.

• no dejó de tocar el claxon
• no paraba de tocar el claxon

Both can mean he wouldn’t stop honking.

A rough nuance:

• no dejó de... = a bit more neutral or formal
• no paraba de... = very common, slightly more vivid or colloquial

Yes, tocar often means to touch or to play an instrument, but it also has the sense of to sound certain devices.

So:

• tocar el claxon = to honk the horn
• tocar la bocina = to honk the horn in other varieties of Spanish

It is an idiomatic use. You do not translate it literally as touch the horn.

Yes, claxon is a normal word in Spain.

A learner should know that vocabulary varies by region:

• In Spain, claxon is very common.
• In many parts of Latin America, people often say bocina, and in some places other words are used too.

Also, it is masculine:

• el claxon

Because ponerse + adjective is a fixed and very common way to say to become / to get a certain state.

So:

• ponerse nervioso/a = to get nervous
• ponerse triste = to get sad
• ponerse contento/a = to become happy

The se is part of that verb structure here. Without it, the meaning would change.

Because nerviosa agrees with mi hermana, which is feminine singular.

Agreement:

• mi hermano se puso nervioso
• mi hermana se puso nerviosa

Spanish adjectives usually match the noun in gender and number.

Se puso nerviosa means she became nervous or she got nervous. It shows a change of state.

Estaba nerviosa means she was nervous. It just describes her state, without focusing on the change.

So in this sentence:

• se puso nerviosa fits well because the honking caused a reaction.

Por eso means for that reason, because of that, or simply so.

It links the cause and the result:

• Cause: the taxi driver kept honking
• Result: the sister got nervous

Similar connectors:

• así que = so
• por lo tanto = therefore
• de modo que = so / therefore

Because Spanish often leaves subject pronouns out when the meaning is clear.

Here:

• El taxista already tells you who did dejó
• mi hermana tells you who se puso nerviosa

Spanish does not need pronouns as often as English does because the verb forms and context usually make the subject clear.

Not always.

In Spanish, a comma before y is often omitted:

• El taxista no dejó de tocar el claxon y por eso mi hermana se puso nerviosa.

That version is also fine.

The comma can be used to mark a pause or make the relationship between the two clauses clearer, especially since the second clause expresses a consequence and has a different subject. So it is not strange here, but it is not absolutely required either.

Taxista means taxi driver and can refer to either a man or a woman.

The form usually stays the same:

• el taxista = male taxi driver
• la taxista = female taxi driver

So the article tells you the gender here:

• El taxista = the male taxi driver