No modelamos aún, sino que primero humedecemos la arcilla.

Because Spanish usually drops subject pronouns when the verb ending already makes the subject clear.

• modelamos = we shape / we model
• humedecemos = we moisten

Both endings clearly show nosotros, so adding nosotros would usually only be for emphasis or contrast.

For example:

• No modelamos aún... = normal
• Nosotros no modelamos aún... = more emphatic, as if contrasting we with someone else

Spanish often uses the present tense to describe a process, give instructions, or explain what happens step by step.

So this kind of sentence can sound like:

• what we are doing now
• what we do at this stage
• how the process works

In English, this is often expressed with we ..., first we ..., or sometimes an imperative-like style such as first, moisten the clay. Spanish often prefers the present tense in explanations like this.

Because no ... sino que ... is the normal structure for correcting a negative idea: not this, but rather that.

Here the idea is:

• not shaping the clay yet
• but instead first moistening it

So sino que is more precise than pero.

Compare:

• No modelamos aún, sino que primero humedecemos la arcilla.
  = We are not shaping it yet; instead, we first moisten the clay.

• No modelamos aún, pero la arcilla ya está lista.
  = We are not shaping it yet, but the clay is already ready.

In the second example, pero just adds contrast. It does not correct the first statement in the same way.

Use sino que when what follows is a full clause with a conjugated verb.

Here, after sino que, we get:

• primero humedecemos la arcilla

That is a full clause, because it contains the conjugated verb humedecemos.

Use sino by itself before a noun, adjective, or infinitive:

• No es barro, sino arcilla.
• No está seco, sino húmedo.
• No quiero modelar, sino esperar.

But with a full clause:

• No modelamos aún, sino que primero humedecemos la arcilla.

Here aún means yet or still.

In standard spelling, aún usually has an accent when it means still / yet, roughly like todavía.

Examples:

• Aún no está listo. = It still is not ready / It is not ready yet.
• No modelamos aún. = We are not shaping it yet.

Without the accent, aun often means something more like even or including:

• Aun los expertos se equivocan. = Even experts make mistakes.

So the accent helps distinguish the meaning.

Yes. Both are correct and natural.

• No modelamos aún
• Aún no modelamos

Both mean essentially the same thing. The difference is mostly one of emphasis and rhythm.

• Aún no modelamos puts yet/still earlier, so it can sound slightly more focused on the fact that the action has not happened yet.
• No modelamos aún sounds very natural too, especially in spoken explanations.

Spanish often allows this kind of flexibility with adverbs.

Because primero is an adverb showing order: first.

Placing it before the verb is a very natural way to mark the sequence of actions:

• primero humedecemos la arcilla

It tells you immediately what stage comes first.

Spanish word order is flexible, so other versions are possible, such as:

• No modelamos aún, sino que humedecemos primero la arcilla.

That is also grammatical, but it shifts the focus slightly. The original version sounds very natural in a step-by-step explanation.

Because Spanish often uses the definite article with a specific thing that is understood from the context.

Here la arcilla means the clay being worked with, the clay already known in the situation.

English often omits the article in places where Spanish keeps it. So even though English might sometimes just say clay, Spanish often says la arcilla when referring to a specific material in the activity.

Here modelar means to shape, to mold, or to sculpt by hand.

It is not the same as every meaning of English to model. In English, to model can also mean things like:

• to be a fashion model
• to represent something mathematically
• to demonstrate behavior

In this clay context, modelar refers to shaping the material into a form.

A few useful points:

• The h is silent.
• In much of Spain, c before e is pronounced like th in think.
• The stress falls on ce: hu-me-de-ce-mos

So in much of Spain it sounds roughly like:

• u-me-de-THE-mos

And modelamos is stressed on la:

• mo-de-LA-mos

If you are learning Peninsular Spanish, that c sound is one of the main pronunciation features to notice.