Some Italian adjectives have different masculine and feminine forms, while others have the same form for both genders in the singular.
Importante is one of the adjectives that has:
So:
In your sentence, importante agrees with impressione, but its singular form stays the same.
They do different jobs in the sentence.
So the structure is:
La prima impressione + è + importante
subject + verb + description
This is similar to English:
Not in normal Italian for this meaning.
The natural phrase is la prima impressione, where prima means first in an ordering sense.
Putting prima after the noun here sounds unnatural or changes the feeling. In this expression, prima normally goes before the noun.
So the standard form is:
A simple pronunciation guide is:
La PREE-ma im-pres-SYO-ne eh im-por-TAN-te
A few notes:
If you want to sound smoother, link the words together naturally: La prima impressione è importante.
Grammatically, it says the first impression because of la.
But in real usage, Italian often uses the definite article where English might use a more general expression. So this sentence can express a general idea, like:
If you wanted to say a first impression more specifically, you could say:
But your sentence is the most natural way to express the general statement.
The sentence has a very simple structure:
So:
La prima impressione | è | importante
subject | verb | adjective
This is a very common sentence pattern in Italian:
Yes. Prima can have different uses in Italian depending on context.
In your sentence, prima is an adjective meaning first:
But prima can also be an adverb meaning before / earlier:
So here, because it comes before a noun and agrees with it, it is clearly an adjective.
Yes, it is completely natural and idiomatic.
La prima impressione è importante is a normal way to express the idea that first impressions matter.
Depending on context, Italians might also say:
But your sentence is clear, correct, and natural.