Αύριο θα ξαναμιλήσω με τη μηχανικό, γιατί θέλω να ρωτήσω και για το λάστιχο.

It is not past tense here. After θα, Greek uses a verb form that looks like the aorist subjunctive for many verbs. So:

• θα ξαναμιλήσω = I will speak again

This is the normal way to form the simple future in Modern Greek.

A useful pattern is:

• θα
  • perfective verb form = future single/complete action
• θα μιλάω / θα μιλήσω can contrast a repeated/ongoing future action vs. a single future action

Here, ξαναμιλήσω fits well because the speaker means one future conversation.

ξανα- means again.

So:

• μιλάω / μιλήσω = speak
• ξαναμιλάω / ξαναμιλήσω = speak again

Greek often builds again directly into the verb with ξανα-.
You can also sometimes express the same idea with ξανά as a separate word:

• θα ξαναμιλήσω
• θα μιλήσω ξανά

Both are natural. The prefixed form is very common.

There are two things going on here:

1. με takes the accusative

   • so you need the object form, not the dictionary/nominative form

2. μηχανικός is one of those nouns whose feminine form is often:

   • nominative: η μηχανικός
   • accusative: τη(ν) μηχανικό

So:

• η μηχανικός = the mechanic/engineer (subject form)
• τη μηχανικό = the mechanic/engineer (object form)

After με, you need the object form, so με τη μηχανικό is correct.

Both are related forms of the feminine accusative article.

• την is the full form
• τη is the shortened form often used before many consonants in everyday Greek

So:

• τη μηχανικό is completely normal
• την μηχανικό is also possible

In modern usage, dropping the final -ν is very common, depending on the sound that follows and on style/register.

The noun μηχανικός / μηχανικό can mean either engineer or mechanic, depending on context.

In this sentence, because the speaker wants to ask about το λάστιχο and that likely refers to a vehicle tire, mechanic is probably the intended meaning.

So the context tells you which English word fits best.

In Greek, verbs like θέλω are usually followed by να + another verb.

So:

• θέλω να ρωτήσω = I want to ask
• literally something like I want that I ask, though that is not how you should translate it

This is a very common Greek pattern:

• θέλω να πάω = I want to go
• μπορώ να έρθω = I can come
• πρέπει να φύγουμε = we must leave

So να is required here.

Both are possible in Greek, but they express slightly different aspect.

• να ρωτήσω = to ask once / to ask as a complete action
• να ρωτάω = to be asking / to ask repeatedly or habitually

Here the speaker means one specific question or one asking event, so να ρωτήσω is the natural choice.

This is a very common Greek distinction:

• perfective form for a single complete action
• imperfective form for repeated, ongoing, or habitual action

Here και means also / too.

It shows that asking about the tire is an additional topic, not the only one.

So the sense is:

• I want to ask also about the tire
• or more naturally in English, I also want to ask about the tire

In Greek, και often appears in places that may sound a little different from English word order, but the idea is the same: it adds something.

για here means about / concerning.

So:

• ρωτάω για κάτι = ask about something
• για το λάστιχο = about the tire

The noun λάστιχο can mean different things depending on context, such as:

• tire
• rubber
• hose
• elastic band

In this sentence, because of the likely context with a mechanic, it most naturally means tire.

Greek often uses commas to separate clauses more readily than English does.

Here the comma marks the break between:

• Αύριο θα ξαναμιλήσω με τη μηχανικό
• γιατί θέλω να ρωτήσω και για το λάστιχο

So it helps show that the second clause gives the reason.

Even when English might omit the comma before because, Greek often keeps it.

Yes. That is perfectly natural.

Compare:

• θα ξαναμιλήσω = I will speak again
• θα μιλήσω ξανά = I will speak again

The meaning is essentially the same. The version with ξανα- attached to the verb is especially common and compact.

No. Greek word order is flexible.

You could also say:

• Θα ξαναμιλήσω αύριο με τη μηχανικό...

Putting Αύριο first gives the time expression a bit more prominence. It sets the scene right away: Tomorrow...

That kind of fronting is very common in Greek.

A rough pronunciation guide is:

• Αύριο → AV-rio
• θα ξαναμιλήσω → tha ksa-na-mi-LI-so
• με τη μηχανικό → me ti mi-ha-ni-KO
• γιατί θέλω να ρωτήσω → ya-TI THE-lo na ro-TI-so
• και για το λάστιχο → ke ya to LAS-ti-ho

A couple of useful pronunciation notes:

• γιατί is stressed on the second syllable: για-ΤΙ
• μηχανικό is stressed on the last syllable: μη-χα-νι-ΚΟ
• λάστιχο is stressed on the first syllable: ΛΑ-στι-χο
• ξ is pronounced like ks

Yes, γιατί can mean both why and because.

This is very common in Modern Greek.

• Γιατί έφυγες; = Why did you leave?
• Έφυγα γιατί κουράστηκα. = I left because I got tired.

The meaning depends on whether it is used in a question or to introduce a reason.
In your sentence, it clearly means because.