No seas tan terco y escucha el consejo de tu hermana.

Because this is a negative command addressed to tú.

In Spanish, negative commands for tú use the present subjunctive, not the normal present tense:

• ser → seas
• No seas = Don’t be

So:

• Eres terco = You are stubborn
• No seas terco = Don’t be stubborn

That is why eres would be wrong here.

Because Spanish forms negative and affirmative commands differently for tú.

• Negative tú command: use the present subjunctive
  • no seas = don’t be
• Affirmative tú command: usually use the third-person singular present form
  • escucha = listen

So the sentence mixes:

• No seas... = negative command
• escucha... = positive command

This is completely normal in Spanish.

Compare:

• Sé amable. = Be nice.
• No seas grosero. = Don’t be rude.
• Escucha. = Listen.
• No escuches. = Don’t listen.

Because tan is the form used before adjectives and adverbs.

Here, terco is an adjective, so Spanish uses:

• tan terco = so stubborn

Compare:

• tan alto = so tall
• tan rápido = so fast
• tan terco = so stubborn

Tanto is used in other structures, usually with nouns or to express quantity:

• tanto dinero = so much money
• tantos libros = so many books

So tanto terco is not correct here.

Yes. Terco changes for gender.

• terco = masculine singular
• terca = feminine singular
• tercos = masculine plural / mixed plural
• tercas = feminine plural

So if you are speaking to a woman, you would normally say:

• No seas tan terca...

If you are speaking to a man, you say:

• No seas tan terco...

Spanish adjectives usually agree with the person they describe.

Terco usually means stubborn, pigheaded, or obstinate.

In this sentence, it is clearly negative: the speaker is criticizing the person and telling them to stop resisting advice.

Depending on context, a similar idea can sometimes be more neutral or even positive in English, like determined, but terco itself usually suggests someone is being difficult and refusing to listen.

Because the thing being listened to here is the advice, not the sister.

• escuchar el consejo de tu hermana = listen to your sister’s advice

The direct object is el consejo.

If you wanted to say listen to your sister, you would say:

• Escucha a tu hermana

There, the object is a person, so Spanish uses the personal a.

So compare:

• Escucha el consejo de tu hermana = Listen to your sister’s advice.
• Escucha a tu hermana = Listen to your sister.

It means your sister’s or from your sister, depending on how you explain it in English.

Literally:

• el consejo de tu hermana = the advice of your sister

But in natural English, we usually say:

• your sister’s advice
• or the advice from your sister

So de often covers ideas that English expresses with ’s or from, depending on context.

Because tu means your when speaking directly to someone informally.

This sentence is addressed to tú, so:

• tu hermana = your sister

Su hermana could mean:

• his sister
• her sister
• your sister (formal usted)
• their sister

So tu is the clear informal choice here.

No. The infinitive is escuchar.

Here:

• escuchar = to listen
• escucha = listen! / he listens / she listens / you listen (formal)

In this sentence, escucha is an affirmative tú command.

That form often looks identical to the él/ella/usted present form, which is why it can be confusing.

Yes. If you were speaking formally to one person, you would use usted commands:

• No sea tan terco y escuche el consejo de su hermana.

Changes:

• seas → sea
• escucha → escuche
• tu → su

If speaking to a woman, you would also usually change terco to terca:

• No sea tan terca y escuche el consejo de su hermana.

People in Spain could definitely say it, but it sounds quite direct and a bit scolding.

The sentence gives two commands:

• Don’t be so stubborn
• listen to your sister’s advice

So it can sound like a reprimand, especially because terco is critical.

A softer version might be something like:

• No seas tan terco y hazle caso a tu hermana.
• Deberías escuchar el consejo de tu hermana.

But the original sentence is perfectly natural if the speaker wants to sound firm.

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

• oír = to hear
• escuchar = to listen

Escuchar usually suggests active attention, which fits this sentence well: the person should pay attention to the advice.

So:

• Oigo un ruido. = I hear a noise.
• Escucha el consejo de tu hermana. = Listen to your sister’s advice.

Here, escuchar is the natural choice because the idea is not just perceiving sound, but actually paying attention to what the sister is saying.

Because in Spanish, as in English, you normally do not put a comma before y when simply joining two elements.

So this is standard:

• No seas tan terco y escucha el consejo de tu hermana.

A comma might appear only in special cases, but not in an ordinary sentence like this one.