Reste comes from rester, which means to remain, to stay, or to keep being.
So:
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:
Since l’argile is like elle, the correct form is reste.
Yes, adjectives normally agree in gender and number with the noun they describe, including after verbs like être or rester.
Here, argile is:
So souple must be feminine singular too.
The useful thing is that souple has the same form in masculine and feminine singular:
So the agreement is there, but you do not see a spelling change in the singular.
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:
The exact interpretation depends on context.
Yes. That sentence is correct, but the meaning is slightly different.
So est gives a simple description, while reste emphasizes that the quality continues.
It can mean both, depending on context.
Souple often means:
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.