Marco è bugiardo.

Because è means is and needs the accent to distinguish it from e, which means and.

• è = is
• e = and

So:

• Marco è bugiardo. = Marco is a liar / Marco is deceitful.
• Marco e bugiardo would be incorrect here.

The accent is very important in writing.

Here, bugiardo is functioning as an adjective or a predicate adjective after the verb essere.

It describes Marco’s character:

• Marco è bugiardo. = Marco is deceitful / lying by nature.

Italian often uses an adjective where English might sometimes prefer a noun phrase like a liar.

In Italian, when you describe someone’s character, profession, nationality, religion, or similar qualities after essere, you often do not use an article.

So:

• Marco è bugiardo. = Marco is deceitful / a liar.

This is a normal pattern in Italian. English often needs a, but Italian frequently does not in this kind of sentence.

Yes. Marco è un bugiardo is also correct.

There is a slight difference in feel:

• Marco è bugiardo = Marco is deceitful / is a lying type of person.
• Marco è un bugiardo = Marco is a liar.

The version with un can sound a little more like you are labeling him as a liar directly. The version without the article can sound a bit more descriptive of his character. In many contexts, though, the difference is small.

Yes. It changes to agree with the person being described.

• masculine singular: bugiardo
• feminine singular: bugiarda
• masculine plural: bugiardi
• feminine plural: bugiarde

Examples:

• Marco è bugiardo.
• Maria è bugiarda.
• Marco e Luca sono bugiardi.
• Maria e Anna sono bugiarde.

A simple pronunciation guide is:

• Marco = MAR-ko
• è = like eh
• bugiardo = roughly boo-JAR-do

A more detailed approximation:

MAR-ko eh boo-JAR-do

The stress in bugiardo falls on -giar-.

Because the sentence uses the verb essere (to be), not avere (to have).

• è = he/she is
• ha = he/she has

Since the sentence describes what Marco is like, Italian uses essere:

• Marco è bugiardo. = Marco is deceitful / a liar.

It can be both.

As an adjective:

• Marco è bugiardo. = Marco is deceitful.

As a noun:

• Marco è un bugiardo. = Marco is a liar.

This is common in Italian: some words can work as either adjectives or nouns depending on the structure.

The normal order is:

Marco è bugiardo.

Italian word order is often similar to English in simple sentences: subject + verb + description.

You could change the order for emphasis in special contexts, but that would be less neutral. For a basic statement, Marco è bugiardo is the standard form.

It is neutral in grammar, but the meaning is quite strong. Calling someone bugiardo means you are saying they are dishonest or that they lie.

So grammatically it is ordinary, but socially it can sound accusatory depending on context.