Je centre la motte d’argile sur la girelle.

Here centre is the present-tense verb form of centrer, which means to center.

So:

• un centre = a center / a centre (noun)
• je centre = I center (verb)

In this sentence, je is the subject, so centre has to be a verb: Je centre... = I center...

Because the infinitive is centrer (to center), and je centre is the present indicative form for I.

This is a regular -er verb:

• je centre
• tu centres
• il/elle/on centre
• nous centrons
• vous centrez
• ils/elles centrent

In context, je centre can mean either:

• I center
• I am centering

French simple present often covers both ideas.

Because j’ is only used before a vowel sound or a silent h.

So you get:

• j’aime
• j’habite

But here the next word is centre, which starts with a consonant sound, so it stays:

• je centre

Not j’centre.

Motte means a lump, clump, or mass of something.

So la motte d’argile is literally:

• the lump of clay
• the clay lump

It is a feminine noun, which is why it takes la:

• la motte

Because this is the preposition de linking two nouns: motte de argile → motte d’argile.

Here argile tells you what the lump is made of:

• une motte d’argile = a lump of clay
• un bloc de pierre = a block of stone
• une tasse de thé = a cup of tea

Since argile starts with a vowel, de becomes d’.

It is not the partitive article here, so you do not say de l’argile in this structure.

Argile means clay.

It is generally treated as feminine in French:

• l’argile
• une argile in some contexts

In your sentence, though, you do not see l’ because it is inside the expression motte d’argile.

La girelle is a pottery term. It refers to the wheel head, turntable, or the rotating part/platform on which the clay is placed.

So in pottery context:

• la girelle = the part of the wheel where the clay sits and turns

It is a specialized word, so even many French learners outside pottery may not know it.

Because sur means on or onto, and the lump of clay is physically placed on the surface of the wheel head.

So:

• sur la girelle = on the wheel head

That is the natural preposition when something is resting on top of a surface.

Because both motte and girelle are feminine singular nouns:

• la motte
• la girelle

French nouns have grammatical gender, so the article must match the noun.

A careful pronunciation is approximately:

Je centre la motte d’argile sur la girelle
/ʒə sɑ̃tʁ la mɔt daʁʒil syʁ la ʒiʁɛl/

A few notes:

• je sounds like zhuh
• centre has a nasal vowel in the first syllable
• d’argile is pronounced smoothly together
• girelle begins with the soft j sound, like the s in measure

A rough English-friendly guide could be:

zhuh SAHN-truh la mot dar-zheel sewr la zhee-RELL

That is only approximate, but it can help at first.

It is in the present indicative.

In context, this kind of sentence could be used:

• to describe what someone is doing right now
• to explain a process step by step
• in a demonstration or instruction

So Je centre la motte d’argile sur la girelle could mean:

• I’m centering the lump of clay on the wheel head
• I center the lump of clay on the wheel head

French uses the simple present more often than English does for ongoing actions.

No, it is very standard French word order:

• Je = subject
• centre = verb
• la motte d’argile = direct object
• sur la girelle = prepositional phrase showing location

So the structure is:

Subject + Verb + Object + Location

That makes this a very useful model sentence for learning basic French syntax.