Questa legge è nazionale.

Because legge is a feminine singular noun in Italian.

• questo = this for masculine singular nouns
• questa = this for feminine singular nouns

So:

• questo libro = this book
• questa legge = this law

The demonstrative has to agree with the noun in gender and number.

In Italian, every noun has a grammatical gender. Legge is simply a feminine noun, so the words connected to it must also be feminine:

• questa legge
• la legge
• una legge

Its plural is leggi, and it stays feminine:

• queste leggi = these laws

You usually learn a noun’s gender together with the noun itself.

Because nazionale is an adjective, not a noun.

In Questa legge è nazionale, the structure is:

• Questa legge = subject
• è = is
• nazionale = adjective describing the subject

So it works like:

• This law is national

You would only use an article if nazionale were being used as a noun in some different sentence, which is not the case here.

In Italian, adjectives often come after the noun, especially when they describe a basic quality or classification.

So:

• legge nazionale = national law

This is very normal Italian word order. English often puts adjectives before nouns, but Italian commonly puts them after.

Also, in your sentence, nazionale comes after è because it is the adjective complement:

• Questa legge è nazionale
• literally: This law is national

This is a very important spelling point.

• è with an accent = is
• e without an accent = and

So:

• Questa legge è nazionale = This law is national
• pane e acqua = bread and water

The accent matters both in writing and meaning.

The accent helps distinguish it from e meaning and.

It also shows that it is the verb form from essere:

• io sono = I am
• tu sei = you are
• lui/lei è = he/she is

So in your sentence, è is the third person singular of essere.

Legge is pronounced roughly like LEJ-jeh.

A few helpful points:

• the gg before e sounds like the dg in judge
• the double consonant gg is held a little longer than in English
• the stress is on the first syllable: LÈG-ge

So it is not pronounced like English leg.

It is pronounced roughly na-tsyo-NA-le.

Helpful details:

• z here sounds like ts
• -zione in related words often sounds like tsyoh-neh, though here the word is nazionale, not nazione
• the stress falls on -na-: nazioNÀle

A careful breakdown is:

• na
• zio
• NA
• le

Yes, it changes for number, but not in separate masculine/feminine singular forms.

Singular:

• nazionale = national

Plural:

• nazionali = national

Examples:

• legge nazionale = national law
• leggi nazionali = national laws
• giornale nazionale = national newspaper
• giornali nazionali = national newspapers

So in the singular, nazionale works for both masculine and feminine nouns.

Yes, but it means something slightly different in use.

• Questa legge è nazionale = This law is national
• La legge è nazionale = The law is national

Questa points to a specific law, often one already being indicated, discussed, or contrasted with another. La legge is more like referring to the law in a known context.

The plural is:

Queste leggi sono nazionali.

Changes:

• questa → queste
• legge → leggi
• è → sono
• nazionale → nazionali

So everything has to match the new plural subject.

Yes, and that is exactly what happens.

Italian often does not need subject pronouns like it when the subject is already expressed by a noun.

So instead of saying something like This law, it is national, Italian simply says:

• Questa legge è nazionale

That is the normal and natural way to say it.

Here it is best understood as a classification or type: the law belongs to the national level.

So nazionale is not necessarily describing a personality trait or temporary condition. It is saying that the law is national rather than, for example:

• regionale = regional
• locale = local
• internazionale = international

This is a very common use of essere + adjective in Italian.