El adjetivo cambia de forma cuando el sustantivo está en plural.

In Spanish, it is very common to use the singular definite article to talk about a whole class of things in a general way. So:

• el adjetivo = adjectives in general
• el sustantivo = nouns in general

This is similar to English sentences like The lion is a dangerous animal, where the lion means the species in general, not just one lion.

Here, estar en plural is a fixed and very common way to say that a word or expression is in the plural form.

So:

• está en plural = it is written/used in the plural
• está en singular = it is written/used in the singular

You may also hear es plural in some contexts, but estar en plural sounds very natural when talking about grammatical form.

Literally, it means changes in form or changes shape/form. In grammar, this means that the adjective changes its ending.

For example:

• rojo → rojos
• bonita → bonitas

So the sentence is saying that the adjective does not always stay the same; its form changes to match the noun.

Because in Spanish, the verb cambiar often uses de when you say what something changes in or from.

So:

• cambiar de forma = to change form
• cambiar de color = to change colour
• cambiar de nombre = to change name

Without de, cambia forma would sound incorrect here.

No. In Spanish, adjectives usually agree with the noun in both:

• number: singular or plural
• gender: masculine or feminine

So an adjective can change for more than one reason.

Examples:

• libro rojo → libros rojos
• casa roja → casas rojas

The sentence only mentions the plural part, but gender agreement matters too.

Usually:

• If the adjective ends in a vowel, add -s
  • alto → altos
  • verde → verdes
• If it ends in a consonant, add -es
  • fácil → fáciles
  • popular → populares

But there are some spelling and accent changes in certain cases, so this is the general rule, not every detail.

For the same reason as el adjetivo earlier in the sentence: it is talking about the noun as a grammatical category in general.

So el sustantivo está en plural means when the noun is in the plural form, not when one specific noun is plural.

In grammar, sustantivo is the precise term for noun. In school grammar, that is the standard word.

Nombre usually means name, not noun, although in some older or simpler teaching contexts you might see nombre used in a grammatical sense. For modern standard grammar, sustantivo is the better word.

This is a very normal Spanish sentence structure:

• El adjetivo = subject
• cambia = verb
• de forma = complement
• cuando el sustantivo está en plural = time/condition clause

Spanish word order is often flexible, but this version is the most straightforward and natural for an explanatory grammar sentence.

Cuando means when. It introduces a clause that explains under what circumstance the adjective changes form.

So:

• El adjetivo cambia de forma = the adjective changes form
• cuando el sustantivo está en plural = when the noun is plural

It connects the main idea to the condition or situation in which it happens.

Yes. Adjetivo is a masculine noun, and its article is therefore el.

Likewise:

• el adjetivo
• el sustantivo

Many Spanish nouns ending in -o are masculine, although there are exceptions. In this sentence, both grammatical terms are masculine singular nouns.

Not naturally in English. A word-for-word translation would sound awkward.

The natural English equivalents are:

• is plural
• is in the plural
• is in the plural form

So even though Spanish says está en plural, English usually says is plural or is in the plural.

Yes. It expresses a general rule: Spanish adjectives usually agree with the nouns they describe. If the noun is plural, the adjective normally becomes plural too.

For example:

• un libro interesante
• dos libros interesantes

The adjective interesante agrees with the noun in number, even though its form looks the same in singular and plural except for the final -s.