L’argile reste souple.

Because argile begins with a vowel sound, la becomes l’ before it. This is called elision.

• la argile → incorrect
• l’argile → correct

So l’ here stands for la, because argile is a feminine noun.

Argile is feminine: l’argile = the clay.

You can tell this in the full form:

• la argile would be the underlying form
• but before a vowel, la becomes l’

So if you ever need another determiner, you would say:

• une argile = a clay
• cette argile = this clay

Reste comes from rester, which means to remain, to stay, or to keep being.

So:

• L’argile est souple. = The clay is soft / flexible.
• L’argile reste souple. = The clay remains soft / stays flexible.

Using reste adds the idea that the clay continues to be soft, instead of just describing it.

Because reste is the present tense form used with il/elle/on.

The subject is l’argile, which is singular, so French uses the third person singular form:

• je reste
• tu restes
• il / elle / on reste
• nous restons
• vous restez
• ils / elles restent

Since l’argile is like elle, the correct form is reste.

Here souple is an adjective describing the subject l’argile, but it comes after the verb because it is used with a linking verb like être, rester, devenir, etc.

Structure:

• subject
  • verb
    • adjective

So:

• L’argile reste souple.
• La porte est ouverte.
• Il devient calme.

In English, this works similarly: The clay remains soft.

Yes, adjectives normally agree in gender and number with the noun they describe, including after verbs like être or rester.

Here, argile is:

• feminine
• singular

So souple must be feminine singular too.

The useful thing is that souple has the same form in masculine and feminine singular:

• masculine singular: souple
• feminine singular: souple
• masculine plural: souples
• feminine plural: souples

So the agreement is there, but you do not see a spelling change in the singular.

Because souple is an adjective, not a noun.

After verbs like être and rester, French usually uses the adjective directly:

• L’argile est souple.
• L’argile reste souple.

You would not say reste la souple or reste une souple, because that would be ungrammatical here.

It can often mean clay in general.

French commonly uses the definite article with materials, abstract ideas, and general categories where English may or may not do the same.

So L’argile reste souple can naturally mean something like:

• Clay remains soft
• or The clay remains soft

The exact interpretation depends on context.

Yes. That sentence is correct, but the meaning is slightly different.

• L’argile est souple. = The clay is soft/flexible.
• L’argile reste souple. = The clay remains soft/flexible.

So est gives a simple description, while reste emphasizes that the quality continues.

A careful pronunciation is approximately:

lar-zheel rest soopl

More precisely in IPA:

/laʁ.ʒil ʁɛst supl/

A few notes:

• l’argile sounds like lar-zheel
• the r in French is the French throat r
• souple has a long oo sound like soup, followed by l
• the final e in souple is not pronounced

It can mean both, depending on context.

Souple often means:

• flexible
• supple
• soft in the sense of not rigid

With argile, the idea is usually that the clay is still malleable or easy to shape, not hard or dry. So flexible or supple may fit the nuance better than just soft.