Le huitième étage est calme.

Because étage is a masculine noun in French. The article has to match the gender and number of the noun, so you say le huitième étage.

If the noun were feminine, you would use la, for example la huitième porte.

Because huit means eight, while huitième means eighth.

French uses an ordinal number here, just like English does:

• huit = eight
• huitième = eighth

So le huitième étage means the eighth floor.

In French, ordinal numbers usually come before the noun they describe.

So you get:

• le premier étage
• le deuxième étage
• le huitième étage

That is the normal word order.

French normally does not elide le before words beginning with huit-.

So you say:

• le huitième
• not l'huitième

This is a small but important spelling pattern to remember.

A useful English approximation is wee-TYEM.

A few points:

• the t is pronounced
• the ending sounds roughly like -yem
• the final e is not pronounced

So huitième is not pronounced like plain huit.

A rough English-style approximation is:

luh wee-TYEM ay-TAZH eh kalm

Word by word:

• Le = luh
• huitième = wee-TYEM
• étage = ay-TAZH
• est = eh
• calme = kalm with a clear l

It is only an approximation, but it is a good starting point.

Est is the singular he/she/it form of être, which means to be.

The subject is le huitième étage, which is singular, so French uses est:

Le huitième étage est calme.

It is already in the correct form for this sentence.

Calme agrees with étage, which is masculine singular. The masculine singular form is calme.

Conveniently, the feminine singular is also spelled calme, so you do not see a difference there. In the plural, you would write calmes.

Not in this exact sentence structure.

When the subject is clearly named, as in le huitième étage, French normally uses est:

Le huitième étage est calme.

C’est calme is possible in a different structure, for example:

• Au huitième étage, c’est calme. = It is quiet on the eighth floor.

So c’est is not wrong in general, but it is not the form used in this exact sentence.

Because this sentence makes the eighth floor itself the subject:

Le huitième étage est calme. = The eighth floor is quiet.

If you say au huitième étage, that means on the eighth floor, and the sentence structure changes:

Au huitième étage, c’est calme. = It is quiet on the eighth floor.

Both are natural, but they do not mean exactly the same thing grammatically.

Not always, especially for American English speakers.

In French buildings, le premier étage is usually the floor above the ground floor. That matches British usage more closely than American usage.

So depending on the building and the variety of English:

• French rez-de-chaussée = ground floor
• French premier étage = first floor above ground

Because of that, French huitième étage may correspond to what some Americans would call the ninth floor.