En caso de que el sustantivo esté en singular, no cambies el verbo al plural.

En caso de que is a more formal expression meaning in case or in the event that. It is very common in instructions, formal writing, and grammar explanations.

You could often use si in everyday Spanish, but it would sound less formal:

• Si el sustantivo está en singular...
• En caso de que el sustantivo esté en singular...

The second version sounds more careful and instructional.

In standard Spanish, especially in careful or formal usage, the normal expression is en caso de que. The de is part of the fixed phrase.

So:

• en caso de que = standard
• en caso que = generally avoided in standard Spanish

For a learner, en caso de que is the safe form to use.

Because en caso de que normally triggers the subjunctive.

Here, the clause is not stating a fact; it is presenting a possible case or condition. That is why Spanish uses:

• esté = present subjunctive not
• está = present indicative

So:

• En caso de que el sustantivo esté en singular... = correct
• En caso de que el sustantivo está en singular... = not standard

When talking about grammatical number, Spanish often uses estar en singular or estar en plural.

That means something like to be in the singular form or to be in the plural form.

You may also hear ser singular, and it can be understood, but estar en singular/plural is very natural in grammar explanations because it focuses on the form being used.

So in this kind of sentence, esté en singular sounds especially idiomatic.

Spanish uses definite articles more often than English, especially when talking about grammar, categories, or things already understood from context.

So:

• el sustantivo = the noun
• el verbo = the verb

In English, you might say if the noun is singular, don’t change the verb to plural without thinking much about the articles. In Spanish, keeping el sounds natural here.

Because negative tú commands use the present subjunctive.

Compare:

• affirmative tú command: cambia
• negative tú command: no cambies

So:

• Cambia el verbo al plural.
• No cambies el verbo al plural.

This sentence is giving an instruction to you informally.

Cambies is the present subjunctive form of cambiar for tú.

In this sentence, that subjunctive form is being used as a negative tú command:

• no cambies = don’t change

So grammatically it is a subjunctive form, but functionally it is an instruction.

Because Spanish often leaves out subject pronouns when the verb ending already shows who the subject is.

Here, cambies already tells you the sentence is aimed at tú. So tú is not necessary.

You could add tú for emphasis, but it would not normally be needed:

• No cambies el verbo al plural.
• Tú no cambies el verbo al plural. = more emphatic

These are two different structures:

• estar en singular/plural = to be in singular/plural
• cambiar algo al singular/plural = to change something to the singular/plural

So:

• el sustantivo esté en singular
• cambies el verbo al plural

The first uses en because it describes a state or form.
The second uses a because it describes changing something to a new form.

Al is the contraction of a + el.

So:

• al plural = a el plural

In practice, it means to the plural or more naturally into the plural form.

So:

• cambiar el verbo al plural = change the verb to the plural

It is not a special grammar term by itself; it is just the normal contraction a + el.

Because the direct object is already stated explicitly: el verbo.

So the sentence says:

• no cambies el verbo al plural

Once the context is clear, Spanish could replace el verbo with lo:

• no lo cambies al plural

Both are possible, but when giving a clear instruction, it is often better to name the thing directly.

Yes. Sustantivo is the standard modern grammatical term for noun in Spain and in most of the Spanish-speaking world.

You may sometimes see or hear nombre in older or more traditional school grammar, but sustantivo is the most precise and safest word to learn.