En este ejercicio, tienes que cambiar el pronombre porque el sujeto está en plural.

En este ejercicio means in this exercise.

Spanish often uses en where English might use in or sometimes leave it out. Here, en sets the context: within this exercise / for this exercise.

• este ejercicio = this exercise
• en este ejercicio = in this exercise

Both are natural ideas, but en este ejercicio sounds more like giving instructions within a task.

Spanish distinguishes different degrees of distance:

• este = this, near the speaker
• ese = that, near the listener or a bit farther away
• aquel = that over there, farther away

In classroom instructions, este ejercicio is the normal way to mean this exercise, the one currently being discussed or the one in front of you.

So:

• este ejercicio = this exercise
• ese ejercicio = that exercise
• aquel ejercicio = that exercise over there / that one further away

Tienes que + infinitive is a very common way to express obligation in Spanish: you have to ...

Here:

• tienes = you have (for tú)
• que cambiar = to change

So tienes que cambiar = you have to change

This is one of the most useful patterns in Spanish:

• tengo que estudiar = I have to study
• tenemos que salir = we have to leave
• tienes que cambiar = you have to change

Because the sentence is addressing you informally, using tú.

The verb tener is conjugated like this in the present:

• yo tengo
• tú tienes
• él/ella/usted tiene
• nosotros/as tenemos
• vosotros/as tenéis
• ellos/ellas/ustedes tienen

So:

• tú tienes que cambiar = you have to change
• usted tiene que cambiar = you have to change (formal)

Since many exercises and textbooks speak directly to the learner in an informal way, tienes is very common.

After tener que, Spanish uses an infinitive.

Pattern:

• tener que + infinitive

So:

• tienes que cambiar
• tengo que estudiar
• tenemos que practicar

You do not conjugate the second verb here. The first verb carries the tense and person, and the second stays in the infinitive.

So tienes que cambias would be incorrect.

Because noun gender in Spanish is not determined only by the ending.

It is true that many nouns ending in -o are masculine and many ending in -a are feminine, but words ending in -e can be either masculine or feminine.

Here, pronombre is masculine, so it takes:

• el pronombre
• un pronombre
• este pronombre

You just have to learn the gender with the noun.

This is a very common question.

In your sentence, porque means because:

• ... porque el sujeto está en plural = ... because the subject is plural

Compare:

• porque = because
• por qué = why

Examples:

• Lo hice porque era necesario. = I did it because it was necessary.
• ¿Por qué lo hiciste? = Why did you do it?

So in this sentence, porque is correct because it introduces the reason.

Spanish usually uses estar with expressions like estar en singular and estar en plural.

So:

• está en plural = is in the plural form / is plural
• está en singular = is in the singular form

This is describing the grammatical state or form of the subject.

You may also hear:

• el sujeto es plural
• el sujeto está en plural

Both can be understood, but estar en plural is a very common classroom/grammar expression when talking about forms.

En plural means in the plural form.

It is used when something is not singular but plural:

• el sujeto está en plural = the subject is plural / the subject is in the plural
• la palabra está en singular = the word is in the singular

This is especially common in grammar explanations and exercises.

For example:

• ellos is plural
• vosotros is plural
• el niño is singular
• los niños is plural

The comma separates the introductory phrase from the main instruction.

• En este ejercicio, = introductory context
• tienes que cambiar el pronombre... = main clause

This kind of comma is very natural in written instructions because it makes the sentence easier to read. You may sometimes see similar sentences without a comma in less careful writing, but the comma is perfectly normal here.

Yes, but the meaning and tone change slightly.

• tienes que cambiar el pronombre = you have to change the pronoun
• hay que cambiar el pronombre = one must / it is necessary to change the pronoun

Tienes que speaks directly to the learner. Hay que is more impersonal and general.

In exercise instructions, both are possible:

• Tienes que completar las frases. = You have to complete the sentences.
• Hay que completar las frases. = The sentences must be completed / One must complete the sentences.

Because el sujeto refers to the grammatical element called the subject, not to how many things it contains.

So el sujeto is singular as a noun phrase meaning the subject.

Then está en plural tells you that this subject has plural number.

For example:

• El sujeto está en singular.
• El sujeto está en plural.

In both cases, sujeto stays singular because you are talking about the subject as one grammatical unit.

Yes. In Spain, if you are speaking informally to more than one person, you would normally use vosotros.

So:

• singular informal: tienes que cambiar
• plural informal in Spain: tenéis que cambiar

The sentence would become:

• En este ejercicio, tenéis que cambiar el pronombre porque el sujeto está en plural.

If the instruction were formal or used in Latin America for several people, you would often get:

• tienen que cambiar

So this sentence uses tú because it is speaking to one learner informally.