Mi padre frenó en seco cuando una bicicleta cruzó la calle.

Mi padre is the normal, neutral way to say my father in Spanish.

• mi + noun = the usual possessive structure
• padre mío is much less common and sounds more emphatic, literary, or emotional

So in an everyday sentence, Mi padre frenó... is exactly what you would expect.

Frenó is the pretérito indefinido (preterite) of frenar: he/she braked.

The preterite is used here because the sentence describes a completed action at a specific moment in the past:

• Mi padre frenó en seco... = My father braked suddenly / came to a sudden stop

This is a one-time event, not a repeated action and not background description, so the preterite fits well.

En seco is a fixed expression. In this sentence, frenar en seco means:

• to brake suddenly
• to stop dead
• to come to an abrupt halt

It does not literally mean something like in dry in this context. You should learn frenar en seco as a chunk.

Examples:

• El coche frenó en seco. = The car stopped dead.
• Se quedó en seco. can mean different things in other contexts, so the exact meaning depends on the expression.

Both relate to stopping, but they are not exactly the same.

• frenar = to brake
• parar = to stop

So Mi padre frenó en seco focuses on the action of applying the brakes.
If you said Mi padre paró en seco, it could also make sense in some contexts, but frenó is more precise for a vehicle or for someone braking suddenly.

Cruzó is also preterite, and it presents the crossing as a complete event.

So the sentence gives two completed past events:

• Mi padre frenó en seco
• cuando una bicicleta cruzó la calle

This sounds like: one event happened, and at that moment another event occurred.

If you said cuando una bicicleta cruzaba la calle, that would use the imperfect and would present the crossing as ongoing background action:

• Mi padre frenó en seco cuando una bicicleta cruzaba la calle.

That is possible too, but it shifts the nuance slightly.

• cruzó = the bicycle crossed
• cruzaba = the bicycle was crossing

Una bicicleta introduces the bicycle as new information: it is a bicycle, not one already known to the listener.

Compare:

• una bicicleta = a bicycle
• la bicicleta = the bicycle

In this sentence, the bicycle is being mentioned for the first time, so the indefinite article una is the natural choice.

Cruzar la calle means to cross the street.

A useful difference from English is that Spanish often says cross the street with a direct object:

• cruzó la calle = crossed the street

You do not need a preposition like across here.

Other examples:

• Cruzó el puente. = He/She crossed the bridge.
• Cruzamos la plaza. = We crossed the square.

Because una bicicleta is a thing, not a specific human person.

The personal a is generally used before specific human direct objects:

• Vi a María.
• Escuchamos al profesor.

But with things, animals in many cases, or non-personal objects, there is usually no a:

• Vi una bicicleta.
• Compré un coche.

In your sentence, una bicicleta is the subject of cruzó, not a direct object anyway, so no personal a would appear there.

The subject is una bicicleta.

Spanish often allows flexible word order, but here the structure is straightforward:

• una bicicleta = subject
• cruzó = verb
• la calle = direct object

So literally:

• a bicycle crossed the street

Sometimes, but the meaning changes.

• cruzó la calle = crossed the street
• pasó = passed / went by / went past

If the idea is specifically that the bicycle went across the street, cruzó la calle is the best choice.

If you said cuando una bicicleta pasó, that would simply mean when a bicycle passed by, which is less specific.

Because Spanish can use the preterite for two punctual events in sequence or in close connection.

Here the sentence presents:

1. the bicycle crossed
2. your father braked suddenly

Even though one triggered the other, both are seen as bounded, completed events.

Spanish does not require one of them to be imperfect just because they happened at nearly the same time. The choice depends on how the speaker wants to present each action:

• preterite = event as a whole
• imperfect = ongoing, habitual, background, descriptive

The verbs are:

• frenó ← infinitive frenar = to brake
• cruzó ← infinitive cruzar = to cross

Both are:

• 3rd person singular
• preterite
• meaning he/she/it did the action

So:

• frenó = he/she braked
• cruzó = he/she/it crossed

Because mi padre is masculine singular, frenó here means my father braked.
Because una bicicleta is singular, cruzó means a bicycle crossed.

Yes, they matter a lot.

• frenó = he/she braked
• cruzó = he/she/it crossed

Without the accent, the form changes:

• freno = I brake / brake
• cruzo = I cross

So the accent marks show that these are preterite third-person singular forms, not present-tense first-person forms.

In this sentence, yes: cuando is followed by the indicative because it refers to something that actually happened in the past.

• Mi padre frenó en seco cuando una bicicleta cruzó la calle.

But cuando can also be followed by the subjunctive when referring to the future or something not yet realized:

• Cuando cruce la calle, te llamaré. = When I cross the street, I’ll call you.

So:

• past real event → usually indicative
• future / not yet completed event → often subjunctive

Yes, it sounds natural in Spain Spanish.

A speaker in Spain would readily understand and use:

• Mi padre frenó en seco...
• una bicicleta cruzó la calle

You might also hear small variations depending on region or style, but the sentence itself is completely standard and idiomatic.