Questo atto è importante.

Questo means this. It is a demonstrative adjective, and it has to agree with the noun it goes with.

Here it matches atto, which is masculine singular, so the form is questo.

Other common forms are:

• questo = this, masculine singular
• questa = this, feminine singular
• questi = these, masculine plural
• queste = these, feminine plural

So:

• questo atto = this act
• questa idea = this idea
• questi atti = these acts
• queste idee = these ideas

Because in Italian, a demonstrative like questo usually takes the place of the article.

So Italian says:

• questo atto

not:

• il questo atto

That works much like English this act, not the this act.

Atto can mean several related things depending on context, such as:

• act
• deed
• action
• a formal act
• a legal or official document
• even an act in a play or opera

So the exact nuance depends on the situation. In a legal, official, or formal context, atto often means something like document, measure, or formal act. In a more general context, it can mean deed or action.

Italian nouns have grammatical gender, and atto is a masculine noun.

That is why it goes with:

• questo and not questa
• importante in its singular form
• the verb è, because the subject is singular

Many nouns ending in -o are masculine, though that is a pattern, not an absolute rule.

Its plural is atti.

Because è is the form of the verb essere meaning is.

The accent is important because it distinguishes è from e:

• è = is
• e = and

So:

• Questo atto è importante = This act is important
• Questo atto e importante would be a spelling mistake

In this sentence, importante is not directly attached to the noun inside the noun phrase. It comes after è because it is a predicate adjective.

Structure:

• Questo atto = subject
• è = is
• importante = important

So the sentence is built like:

• This act is important

If you wanted the adjective directly next to the noun, you would say something like:

• un atto importante = an important act

Both are normal, but they are different structures.

Yes, but only partly.

In the singular, importante is the same for masculine and feminine:

• atto importante
• idea importante

In the plural, it becomes importanti:

• atti importanti
• idee importanti

So in this sentence, importante is singular because atto is singular.

A key point is the double t in atto. In Italian, double consonants are pronounced more strongly or held a little longer than single ones.

So atto is not pronounced like a quick English ato. The tt matters.

A rough pronunciation guide for the whole sentence is:

KWES-to AT-to eh im-por-TAN-te

A few notes:

• questo has stress on the first syllable: KWES-to
• atto has stress on the first syllable: AT-to
• è sounds like eh
• importante has stress on tan: im-por-TAN-te

Yes, and many learners notice this.

Before a singular noun starting with a vowel, Italian very often uses the shortened form quest':

• quest'atto

That form is very common and often sounds more natural in everyday Italian before a vowel.

You may also come across questo atto, as in your sentence. It is understandable, but quest'atto is the form you should expect to see very often.

The plural would be:

Questi atti sono importanti.

Here is what changes:

• questo → questi
• atto → atti
• è → sono
• importante → importanti

So:

• Questo atto è importante. = This act is important.
• Questi atti sono importanti. = These acts are important.

Questo atto è importante is the most neutral, standard order.

Italian can sometimes change word order for emphasis, for example:

• È importante questo atto.

That can sound more emphatic or stylistically marked, depending on context. But if you are learning the basic pattern, Questo atto è importante is the safest and most natural version to use first.