Όταν πάω στο σούπερ μάρκετ, προσπαθώ να μην ξοδεύω άσκοπα.

Όταν means when. It introduces a time clause:

Όταν πάω στο σούπερ μάρκετ = When I go to the supermarket

Depending on context, όταν can also feel like whenever or every time that, so this sentence can naturally describe a repeated habit.

This is about aspect, which is very important in Greek.

• πάω presents the trip as a whole event
• πηγαίνω presents it more as an ongoing or habitual action

So:

• Όταν πάω στο σούπερ μάρκετ = when I go / when I make a trip to the supermarket
• Όταν πηγαίνω στο σούπερ μάρκετ = when I go / whenever I am going to the supermarket, with a slightly more habitual feel

In everyday speech, both can be natural here. The version with πάω treats each supermarket visit as a complete occasion.

It is a non-past form. In practical terms, you can understand όταν πάω as when I go.

A useful thing for learners is that πάω is also the form you see after να, as in να πάω. After words like όταν, Greek often uses this same kind of form to refer to a future or repeated event.

So you do not need to overthink the label at first: here it simply means when I go.

Στο is a contraction of σε + το.

• σε = to / in / at
• το = the

So στο σούπερ μάρκετ means to the supermarket here.

Because the verb is πάω (I go), English naturally uses to. With other verbs, σε can also mean in or at.

Greek usually drops subject pronouns when they are not needed.

The verb endings already show the subject:

• πάω = I go
• προσπαθώ = I try

So Greek does not need εγώ unless you want emphasis or contrast:

• Εγώ προσπαθώ να μην ξοδεύω άσκοπα = I try not to spend unnecessarily

That sounds more emphatic.

After προσπαθώ (I try), Greek normally uses να + verb.

So:

• προσπαθώ να μην ξοδεύω = I try not to spend

This is one of the big differences from English. English often uses an infinitive, as in I try not to spend, but Modern Greek usually uses να + a finite verb instead of an infinitive.

Because the verb is inside a να-clause.

Greek uses:

• δεν for ordinary statements
  
  • Δεν ξοδεύω πολύ = I do not spend much
• μην with να, commands, and similar structures
  
  • να μην ξοδεύω = not to spend / that I not spend

So after προσπαθώ να..., the correct negative is μην.

Again, this is about aspect.

• ξοδεύω = imperfective, repeated or general spending
• ξοδέψω = perfective, a single complete act of spending

In this sentence, the speaker means a general habit:

I try not to spend unnecessarily

That is why ξοδεύω fits well.

Compare:

• Προσπαθώ να μην ξοδεύω άσκοπα.
  = I try not to spend unnecessarily in general

• Προσπαθώ να μην ξοδέψω πάνω από 50 ευρώ.
  = I’m trying not to spend more than 50 euros on this occasion

Άσκοπα means unnecessarily, pointlessly, or for no good reason.

It comes from the adjective άσκοπος, meaning useless or pointless.

So:

• ξοδεύω άσκοπα = spend unnecessarily / waste money

It is an adverb here, describing how the spending happens.

Because it is a loanword from English, written in Greek letters:

• σούπερ μάρκετ = supermarket

This expression is very common in Modern Greek. It is usually treated as indeclinable, so its form usually does not change.

Yes.

• όταν = when
• όποτε = whenever

So:

• Όταν πάω στο σούπερ μάρκετ... = When I go to the supermarket...
• Όποτε πάω στο σούπερ μάρκετ... = Whenever I go to the supermarket...

In many contexts, όταν can already imply whenever, but όποτε makes that repeated meaning more explicit.

Yes, that is also natural.

The difference is mostly one of nuance:

• Όταν πάω = when I go, viewing each visit as a complete event
• Όταν πηγαίνω = when/whenever I go, with a more habitual or ongoing feel

In many real-life situations, the difference is small, and both versions would be understood easily.

Because the sentence begins with a time clause:

Όταν πάω στο σούπερ μάρκετ, ...

Greek, like English, normally separates that introductory clause with a comma.

So the structure is:

• time clause
  • comma + main clause

That makes the sentence easier to read.