Eu modelo, ao passo que tu torneias.

That is true: Portuguese often leaves subject pronouns out because the verb ending already shows the person.

So you could also say:

Modelo, ao passo que torneias.

In this sentence, eu and tu are included mainly for contrast and clarity:
I do one thing, you do another.

That fits especially well with ao passo que, which sets up a contrast between two clauses.

Ao passo que is a linking expression meaning whereas, while, or by contrast.

It is used to compare two actions, situations, or qualities, often with a clear sense of opposition or contrast.

In this sentence, it connects:

• Eu modelo
• tu torneias

So the idea is: I do this, whereas you do that.

In European Portuguese, ao passo que sounds a bit more formal or written than very everyday alternatives.

Modelo is the 1st person singular present indicative form of the verb modelar.

So:

• modelar = infinitive, to model / to shape
• eu modelo = I model / I shape

The ending -o is the normal present-tense ending for many verbs with eu.

So the structure is:

• eu modelo = subject + conjugated verb

You would not use the infinitive modelar here, because the sentence needs a fully conjugated verb.

Torneias is the 2nd person singular present indicative form of tornear.

So:

• tornear = infinitive
• tu torneias = you turn / you shape on a lathe

With tu, many -ar verbs take the ending -as in the present tense:

• tu falas
• tu trabalhas
• tu modelas
• tu torneias

Here the stem is tornei- + -as, giving torneias.

In European Portuguese, tu is very common for informal singular you.

So this sentence uses the tu form:

• tu torneias

If you used você, the verb would change to the 3rd person singular:

• você torneia

That would sound more formal, more distant, or depend on regional/social usage.

So for Portugal specifically:

• tu = very common in informal speech
• você = possible, but often less natural in many contexts

Not exactly.

Both can sometimes be translated as while, but they are not identical.

• ao passo que strongly emphasizes contrast
• enquanto can simply mean while in a time sense, and only sometimes implies contrast

Compare the feel:

• Eu modelo, ao passo que tu torneias.
  = a clear contrast: whereas you...

• Eu modelo, enquanto tu torneias.
  = could mean I model while you turn, possibly more simultaneous than contrastive

So ao passo que is the better choice when the point is comparison or opposition.

Because the sentence contains two clauses joined by a linking expression:

• Eu modelo
• ao passo que tu torneias

The comma helps separate the two contrasted parts clearly. With a connector like ao passo que, the comma is natural and standard.

Yes. Portuguese often allows that.

You could say:

Modelo, ao passo que torneias.

That would still be grammatical, because the verb endings already show who is doing each action:

• modelo = I
• torneias = you with tu

But the version with eu and tu is useful when you want to stress the contrast between the two people.

It is the present indicative.

• eu modelo
• tu torneias

This tense can express:

• what someone does now
• a habitual action
• a general fact

So depending on context, it could mean:

• I am modelling, whereas you are turning
• I model, whereas you turn
• I do modelling, whereas you do turning

Portuguese present tense is flexible in this way, just like English can be in some contexts.

Yes, modelo can also be a noun, meaning model.

But here it is clearly a verb, because:

• it follows the subject eu
• it is paired with another conjugated verb, torneias
• the sentence structure is that of two clauses with actions

So in this sentence:

• eu modelo = I model / I shape

not

• eu modelo = I model as a noun phrase, which would not make sense here

Grammatically, yes. But stylistically, it sounds somewhat formal, technical, or literary, mainly because of ao passo que.

In everyday speech, many speakers might choose simpler connectors such as:

• mas
• enquanto
• or just two short sentences

For example, something more conversational might be:

Eu modelo, mas tu torneias.

Still, the original sentence is perfectly good Portuguese, especially in written or more careful language.

They are both regular -ar verbs.

That means they follow the normal present-tense pattern:

• eu modelo / torneio
• tu modelas / torneias
• ele/ela modela / torneia
• nós modelamos / torneamos
• vós modelais / torneais
• eles/elas modelam / torneiam

In your sentence, only two forms appear:

• modelo
• torneias

So it is a good example of standard -ar verb conjugation.

Yes.

In Portuguese, tu and você take different conjugations:

• tu torneias
• você torneia

This is very important for learners, because English uses the same verb form for you, but Portuguese does not.

So whenever you see tu, expect the 2nd person singular verb form.