Je mets le moule sur la table avant de préparer la pâte.

Because mettre is an irregular verb.

In the present tense, it goes:

• je mets
• tu mets
• il / elle / on met
• nous mettons
• vous mettez
• ils / elles mettent

So je mets is simply the correct first-person singular form of mettre.

It is the present tense.

In French, the present tense often covers both ideas:

• I put
• I am putting

So Je mets le moule sur la table can mean either, depending on context.

French does not usually need a separate am putting form the way English does.

French uses articles much more often than English.

So where English might say:

• I put the pan on the table before making batter
• or sometimes even omit an article in other contexts

French normally keeps the article:

• le moule
• la pâte

Here, the definite articles make the sentence sound natural. In everyday French, leaving them out would usually be wrong.

Un moule is a mold, tin, or baking pan, depending on context.

In a cooking sentence like this, it usually means something like:

• a cake pan
• a baking mold
• a tin

So the exact English word depends on what kind of container is being used.

After avant, French normally uses de before an infinitive.

So:

• avant de préparer = before preparing / before making

You cannot normally say avant préparer.

A useful pattern to remember is:

• avant de + infinitive

Examples:

• avant de partir = before leaving
• avant de manger = before eating
• avant de préparer = before preparing

Because after avant de, French uses the infinitive when the subject is the same as in the main clause.

Here, the subject of both actions is je:

• Je mets...
• avant de préparer...

That means I put the mold on the table, and I prepare the batter.

So French uses:

• avant de + infinitive

If the subject changed, French would usually use a full clause instead, often with avant que.

For example:

• Je mets le moule sur la table avant qu’il prépare la pâte.

Not in this sentence as it stands.

Use avant de + infinitive when the subject is the same:

• Je mets le moule sur la table avant de préparer la pâte.

Use avant que + verb when the subject is different:

• Je mets le moule sur la table avant qu’elle prépare la pâte.

Also, after avant que, French normally uses the subjunctive:

• avant qu’elle prépare

So avant de is the correct choice here because the same person does both actions.

Yes, sur la table is in a very normal place.

The basic structure is:

• Je mets
  • object + place
• Je mets le moule sur la table

That is the most natural order.

You could move things around for emphasis in some contexts, but this version is the straightforward, neutral one.

They are different words:

• la pâte = dough, batter, paste
• les pâtes = pasta

So in this sentence, la pâte means the mixture being prepared for baking or cooking.

Also note the accent:

• pâte has a circumflex accent: â

That accent is part of the spelling.

It is pronounced approximately like:

• zhuh may

A few helpful points:

• Je sounds like zhuh.
• mets sounds like may.
• The final -s in mets is not pronounced.

So even though mets is spelled with s, you do not hear that s when speaking.

Yes, you often could, but the nuance is a little different.

• mettre = to put, place
• poser = to set down, place down

In this sentence, mettre is the more general, flexible verb.
Poser can sound a bit more specifically like setting something down onto a surface.

So both may work, but je mets is a very common neutral choice.