En la frase “la casa grande”, “grande” es un adjetivo que no cambia en singular.

In Spanish, descriptive adjectives usually come after the noun.

So the normal word order is:

• la casa grande = the big house

Putting the adjective before the noun is sometimes possible, but it often changes the tone or meaning. In neutral everyday Spanish, la casa grande is the standard order.

It means that grande has the same form with both masculine and feminine singular nouns.

For example:

• el libro grande = the big book
• la casa grande = the big house

Even though libro is masculine and casa is feminine, the adjective stays grande in both cases.

Yes. It does not change for gender in the singular, but it does change for number.

Examples:

• singular: la casa grande
• plural: las casas grandes

So grande becomes grandes in the plural.

It can also become gran before a singular noun in some contexts:

• una gran casa

That is a special shortened form.

Because casa is a feminine noun in Spanish, so it takes the feminine singular article la.

• la casa
• not el casa

The adjective grande agrees with the noun too, but in this case its feminine singular form looks exactly the same as its masculine singular form.

No. Some adjectives change for gender, and some do not.

For example:

Adjectives that change for gender

• alto / alta
• bonito / bonita

Examples:

• el chico alto
• la chica alta

Adjectives that do not change for gender

• grande
• interesante
• fácil

Examples:

• el libro interesante
• la película interesante

So grande belongs to the group of adjectives that keep the same form for masculine and feminine singular.

It still agrees with the noun grammatically, but the agreement is not always visible in the singular.

With grande, masculine singular and feminine singular are both grande, so you do not see a difference.

You can see agreement more clearly in the plural:

• el libro grande
• la casa grande
• los libros grandes
• las casas grandes

So it agrees in number, and in singular it is simply one of those adjectives whose masculine and feminine forms are identical.

It is generally not the normal way to say the big house in everyday Spanish.

The usual expression is:

• la casa grande

If you put the adjective before the noun, it can sound literary, emphatic, or like it has a different nuance. Also, with grande, Spanish often prefers the shortened form gran before a singular noun:

• una gran casa

But gran casa usually means something more like a great/impressive house, not just a large house.

So for simple description, use la casa grande.

Gran is a shortened form of grande used before a singular noun.

Examples:

• una gran casa
• un gran libro

Very often, gran suggests great, important, or impressive, not only physical size.

Compare:

• una casa grande = a big house / a large house
• una gran casa = a great/impressive house

So they are related, but they are not always interchangeable.

Singular means one person, thing, or item.

Examples:

• la casa = singular
• las casas = plural

So when the sentence says grande does not change in singular, it means that with one masculine noun or one feminine noun, the form stays grande.

It becomes:

• las casas grandes

Here all parts show plural:

• la → las
• casa → casas
• grande → grandes

So even though grande does not change for gender in the singular, it still changes for number.

Yes. In la casa grande, grande is a regular descriptive adjective modifying casa.

It tells you a quality of the noun:

• casa = the noun
• grande = the adjective describing it

So the sentence is pointing out a grammar fact: grande is an adjective whose singular form is the same for masculine and feminine nouns.