Marie est calme et posée.

Because Marie is third person singular: Marie = she.

The verb here is être (to be):

• je suis = I am
• tu es = you are
• il/elle est = he/she is

So Marie est calme et posée means Marie is calm and composed.

Because French adjectives usually agree with the noun they describe.

Marie is feminine, so the adjective must be feminine singular too:

• masculine: posé
• feminine: posée

That extra -e marks the feminine form.

Some French adjectives have the same form in the masculine and feminine singular.

So:

• un homme calme
• une femme calme

Both use calme.

By contrast, posé / posée changes form.

Here, posée is an adjective, not a past participle being used as part of a tense.

In this sentence, calme and posée both describe Marie’s character or manner after the verb est.

So the structure is:

• Marie = subject
• est = verb
• calme et posée = adjectives describing the subject

They are similar, but not identical.

• calme = calm, not agitated
• posée = composed, self-possessed, measured, steady

So together they give a fuller description. It suggests Marie is not only calm, but also collected and thoughtful in the way she behaves.

Because after être (to be), French commonly puts adjectives after the verb, just like English:

• Marie est calme.
• Marie is calm.

This is a normal pattern: subject + être + adjective.

No. After être, French uses adjectives directly.

So you say:

• Marie est calme
• not Marie est une calme
• not Marie est la posée

The words calme and posée are adjectives, not nouns.

A simple English-friendly approximation is:

mah-REE eh calm ay poh-ZAY

A few details:

• Marie = mah-REE
• est = eh
• calme = roughly calm (but with a French l sound)
• et = ay
• posée = poh-ZAY

Also, the final -e in posée is not pronounced on its own; it is there mainly to mark the feminine form.

The accent in é helps show the pronunciation: it sounds like ay in say.

So:

• posé = poh-ZAY
• posée = also pronounced poh-ZAY

The added feminine -e changes the spelling, but not really the pronunciation here.

Yes. When describing a person, posé / posée often means someone is:

• composed
• level-headed
• measured
• self-possessed

So Marie est posée sounds natural and means Marie has a calm, settled manner.

Yes, only the adjective that shows feminine agreement would change.

For a man, you would say:

Marc est calme et posé.

Compare:

• Marie est calme et posée.
• Marc est calme et posé.

calme stays the same, but posée becomes posé.

In the plural, both adjectives take plural agreement:

• Marie et Anne sont calmes et posées.
• Marc et Paul sont calmes et posés.

So:

• singular feminine: calme et posée
• singular masculine: calme et posé
• plural feminine: calmes et posées
• plural masculine or mixed: calmes et posés