È una decisione legale.

Because è is the third-person singular form of essere (to be) in the present tense: he/she/it is.

The accent is important because it distinguishes è (is) from e (and).

• è = is
• e = and

At the beginning of the sentence, it is capitalized, so you see È.

Italian often omits subject pronouns when they are clear from the verb form. So È by itself can mean:

• it is
• this is
• that is
• sometimes he/she is, depending on context

In this sentence, the exact subject is not stated, because the context already makes it clear.

Because decisione is a feminine singular noun, and the indefinite article must agree with it.

So:

• un = masculine singular
• una = feminine singular

Since decisione is feminine, you say una decisione.

You usually learn the gender of each noun, but there is also a helpful pattern: many Italian nouns ending in -ione are feminine.

Examples:

• la decisione
• la nazione
• la situazione
• la lezione

So decisione takes feminine articles and adjectives where needed.

In Italian, most adjectives normally come after the noun they describe.

So:

• una decisione legale = a legal decision

This is the most natural order here.
While some adjectives can come before the noun, that is much more common with certain descriptive adjectives, and legale is normally placed after the noun.

It actually does agree, but adjectives ending in -ale often have the same singular form for masculine and feminine.

So:

• masculine singular: legale
• feminine singular: legale
• masculine plural: legali
• feminine plural: legali

That means in the singular, both genders look the same.

Examples:

• un documento legale
• una decisione legale

But in the plural:

• documenti legali
• decisioni legali

Normally, no. In this sentence, decisione is a singular countable noun, so you need an article:

• È una decisione legale. ✅
• È decisione legale. ❌

Italian usually requires the article here, just as English requires a in It is a legal decision.

Usually it matches English legal very closely, but you should still rely on context. In this sentence, legale means related to the law or lawful in a legal sense, depending on context.

So una decisione legale is a decision that is legal or law-related.
In real usage, Italian sometimes distinguishes between slightly different legal ideas using other words too, but legale is the normal straightforward adjective here.

A natural pronunciation is roughly:

eh OO-nah dee-chee-ZYO-neh leh-GAH-leh

A few notes:

• È sounds like an open e, similar to eh
• decisione has the stress on -o-: de-ci-sió-ne
• legale has the stress on -ga-: le-gà-le

So the rhythm is:

È una decisione legale.

Everything that needs to agree in number changes:

• È una decisione legale. = singular
• Sono decisioni legali. = plural

Changes:

• È → Sono
• una decisione → decisioni
• legale → legali

Notice that in the plural, the indefinite article often disappears in Italian in this kind of sentence.