No sé el motivo de su enfado, pero quiero hablar con ella con calma.

Because sé is the yo form of saber in the present tense: yo sé = I know.

The accent distinguishes it from se, which is a pronoun used in many other structures, for example:

• Se llama Ana.
• Se vendieron todas las entradas.

So in No sé..., the accent is necessary.

Spanish usually uses saber for knowing information, facts, or answers, and conocer for being familiar with people, places, or things.

Here, the speaker does not know the reason for the anger, so saber is the natural verb:

• No sé el motivo... = I don’t know the reason...
• No sé por qué está enfadada. = I don’t know why she is angry.

You would use conocer in sentences like:

• No conozco a su hermana.
• No conozco ese lugar.

Because Spanish often uses the definite article where English might or might not use the.
Here, el motivo means the reason.

So:

• No sé el motivo de su enfado = I don’t know the reason for her anger.

You could also say:

• No sé por qué está enfadada.

That version is a little more direct and conversational, while el motivo de su enfado sounds slightly more formal or abstract.

It is a noun phrase built like this:

• el motivo = the reason
• de = of / for
• su enfado = her anger / his anger / your anger / their anger

So literally it is the reason for/of her anger.

The preposition de links motivo to what the reason is connected with. This is very common in Spanish:

• la causa del problema
• el origen de la discusión
• la razón de su decisión

Not necessarily. Su can mean:

• his
• her
• your (formal singular)
• their

So su enfado is grammatically ambiguous on its own.

However, in this sentence, the later phrase hablar con ella strongly suggests that su refers to her. So the most natural interpretation is:

• I don’t know the reason for her anger, but I want to talk to her calmly.

If you wanted to make it clearer, Spanish can add clarification:

• ...de su enfado, el de ella...
• ...de su enfado, de María...

Because enfado is a noun, while enfadado / enfadada is an adjective.

• enfado = anger, annoyance
• enfadado / enfadada = angry

So:

• su enfado = her anger
• está enfadada = she is angry

Both are correct, but they work differently in the sentence:

• No sé el motivo de su enfado
• No sé por qué está enfadada

These two ideas are very similar.

Yes. Enfado and enfadarse are very common in Spain.

In many parts of Latin America, learners may hear enojo or enojarse more often:

• Spain: Está enfadada. / No entiendo su enfado.
• Many Latin American varieties: Está enojada. / No entiendo su enojo.

Since you are learning Spanish from Spain, enfado is a very natural choice.

Because hablar is normally used with con when you mean to talk with/to someone:

• hablar con alguien

So:

• Quiero hablar con ella. = I want to talk to her.

Using la directly after hablar is not the normal pattern for this meaning.

Compare:

• Hablé con Ana ayer.
• Quiero hablar con mi jefe.

Spanish often uses the preposition con where English simply uses to.

After a preposition such as con, Spanish uses a prepositional pronoun, not a direct object pronoun.

So:

• con ella = with her
• con él = with him
• con nosotros = with us

By contrast, la is a direct object pronoun:

• La veo. = I see her.
• La conozco. = I know her.

But after con, you need ella:

• Quiero hablar con ella.

Con calma means something like:

• calmly
• in a calm way
• without rushing or arguing
• when things are calmer, depending on context

In this sentence, it suggests the speaker wants to talk in a peaceful, measured way:

• Quiero hablar con ella con calma.

It is a very common expression in Spanish. You can also hear:

• Hazlo con calma.
• Vamos a hablar con calma.
• Piénsalo con calma.

It often carries the idea of taking things slowly and carefully, not just being emotionally calm.

Yes, absolutely. It is completely natural.

The two con phrases do different jobs:

• con ella = with her
• con calma = calmly / in a calm way

So the repetition is not a problem. Spanish does this often when different prepositional phrases are needed.

You could also slightly restructure the sentence for style, for example:

• ...pero quiero hablar con ella tranquilamente.
• ...pero quiero hablar tranquilamente con ella.

But the original version is perfectly normal.

Yes. Tranquilamente is a good alternative.

Compare:

• Quiero hablar con ella con calma.
• Quiero hablar con ella tranquilamente.

Both are natural.
Con calma often suggests taking a calm, careful approach, while tranquilamente can sound a bit more like peacefully/calmly in manner. In many contexts, though, the difference is very small.

Because the speaker is expressing a current wish or intention:

• quiero hablar = I want to talk

The sentence has two present-tense verbs:

• No sé = I don’t know
• quiero = I want

That is very normal. The speaker first states a lack of knowledge, then a present intention:

• I don’t know the reason for her anger, but I want to talk to her calmly.

Yes. The original sentence is natural, but slightly polished because of el motivo de su enfado. A more everyday version might be:

• No sé por qué está enfadada, pero quiero hablar con ella con calma.

This sounds a bit more direct and conversational.

So the difference is roughly:

• No sé el motivo de su enfado... = slightly more formal or abstract
• No sé por qué está enfadada... = more everyday and straightforward