L’éponge de potier lisse la plaque de terre.

Because French normally shortens le or la to l’ before a vowel sound. This is called elision.

• la éponge → l’éponge

The full article here is la, because éponge is a feminine noun. The apostrophe does not change the gender; it only avoids a clash of sounds.

Here, lisse is a verb: the third-person singular present form of lisser, meaning to smooth.

So the structure is:

• L’éponge de potier = subject
• lisse = verb
• la plaque de terre = direct object

This can confuse learners because lisse is also an adjective meaning smooth. But in this sentence it must be the verb, because it sits between the subject and the object.

Compare:

• La plaque est lisse. = The slab is smooth.
  Here lisse is an adjective.
• L’éponge lisse la plaque. = The sponge smooths the slab.
  Here lisse is a verb.

French does not use an apostrophe-s possessive like English. Instead, it often uses a de phrase.

But here de potier is not really simple ownership. It describes the type or function of the sponge: a sponge associated with pottery or used by a potter.

So:

• éponge de potier = potter’s sponge / pottery sponge

If you said l’éponge du potier, that would usually mean the potter’s sponge, in the sense of the sponge belonging to the potter.

So the difference is roughly:

• éponge de potier = kind/type
• éponge du potier = belonging to a specific potter

Because French often leaves out the article after de when the second noun describes:

• type
• material
• function
• category

That is what happens in both phrases here:

• éponge de potier = a potter-type sponge / sponge for pottery
• plaque de terre = a slab of clay/earth

This is very common in French. Similar patterns are:

• une robe de soie = a silk dress
• une statue de bronze = a bronze statue
• une salle de classe = a classroom

If you add an article, the meaning usually becomes more specific:

• du potier = of the potter
• de la terre = of the earth/soil, or from the ground, depending on context

Because plaque is a feminine singular noun, so it takes the feminine singular definite article la.

• la plaque

The article agrees with plaque, not with the word after it.

Also, la plaque de terre is the direct object of the verb lisse.

In a normal French statement, the usual order is:

• subject + verb + object

That is exactly what we have here:

• L’éponge de potier = subject
• lisse = verb
• la plaque de terre = object

So the sponge is the thing doing the smoothing.

This is the same basic word order as in English, which makes the sentence relatively straightforward grammatically.

Not always. De is very flexible in French, and English translations depend on context.

In this sentence:

• de potier does not mean a literal of potter in natural English. It works more like potter’s or for pottery.
• de terre indicates the material/substance, so it means something like of clay or earthen in context.

So while de often corresponds to of, that is not always the best English equivalent. Very often it helps link nouns in a way that English expresses with:

• possession
• material
• type
• purpose

A careful pronunciation is approximately:

lay-ponzh duh poh-tyay lees la plak duh tair

A more IPA-style version is:

/le.pɔ̃ʒ də pɔ.tje lis la plak də tɛʁ/

A few helpful notes:

• L’éponge: the on sound is nasal.
• potier sounds roughly like poh-tyay.
• lisse sounds like leece.
• terre ends with a pronounced French r.

There is no special extra liaison to worry about beyond the normal flow of the sentence.