Il sostantivo è singolare, ma l’aggettivo è plurale.

Because Italian usually drops the final vowel of lo or la before a word that begins with a vowel. This is called elision.

So:

• la + aggettivo → l’aggettivo

Even though aggettivo is a masculine noun, the article used here is historically from lo, which also becomes l’ before vowels:

• lo amico → l’amico
• lo aggettivo → l’aggettivo

In modern Italian, before a masculine singular noun starting with a vowel, you normally use l’.

The choice depends on the sound that follows.

• sostantivo begins with s, so it takes il: il sostantivo
• aggettivo begins with a vowel, so it takes l’: l’aggettivo

So the articles are not different because the words mean different things, but because the next word begins with a different sound.

Because è is the verb essere in the he/she/it is form, and in Italian it must be written with a grave accent.

• è = is
• e = and

This accent is very important, because without it the word becomes a different word entirely.

So:

• Il sostantivo è singolare = The noun is singular
• Il sostantivo e singolare would be incorrect

Because they are describing singular nouns:

• singolare describes il sostantivo
• plurale describes l’aggettivo

Both sostantivo and aggettivo are singular masculine nouns, so the adjectives that refer to them are also singular masculine.

This is a point that often confuses English speakers: plurale does not become plural just because it means plural. It stays singular because it is describing one adjective.

Compare:

• L’aggettivo è plurale = the adjective is plural
• Gli aggettivi sono plurali = the adjectives are plural

Because here they are being used as predicate adjectives after the verb essere.

Italian normally does not use an article in this structure:

• Il libro è interessante
• La casa è grande
• Il sostantivo è singolare
• L’aggettivo è plurale

You use the article with the noun, but not with the adjective that comes after essere.

Ma means but. It connects two contrasting ideas:

• Il sostantivo è singolare
• l’aggettivo è plurale

Italian uses ma much like English uses but. It usually goes between the two clauses.

So the structure is:

• statement 1 + ma
  • statement 2

This is a very normal and natural position for ma.

Italian generally prefers to repeat the verb when there are two full clauses joined by ma.

So this sounds natural:

• Il sostantivo è singolare, ma l’aggettivo è plurale.

You might understand the sentence even if the second è were omitted in some contexts, but repeating it is clearer and more standard.

It works just like English:

• The noun is singular, but the adjective is plural.

No. Sostantivo is a correct and common grammatical term, but you may also see nome used in grammar explanations, especially in school-style grammar.

So both can refer to noun, depending on context:

• sostantivo
• nome

However, in more formal grammatical language, sostantivo is very common.

Yes, but the original order is the most neutral and straightforward.

For example, you could say:

• L’aggettivo è plurale, ma il sostantivo è singolare.

This changes the emphasis, because now the sentence starts with the adjective instead of the noun.

Italian word order is somewhat flexible, but the original version is the clearest if the topic starts with the noun.

Because in this sentence, aggettivo is not functioning as an adjective modifying sostantivo. It is a noun meaning adjective.

So the sentence is talking about two grammar terms:

• il sostantivo = the noun
• l’aggettivo = the adjective

This is not a phrase like un aggettivo plurale modifying a noun in normal grammar. Instead, it is a statement about grammatical categories.

That is why each part has its own subject and its own description:

• the noun is singular
• the adjective is plural

Yes. When l’ is used before a vowel, the apostrophe is required in normal spelling.

So you must write:

• l’aggettivo

not:

• laggettivo
• l aggettivo
• il aggettivo

The apostrophe shows that the original article has been shortened before a vowel.