Όταν περνάω από το κρεοπωλείο ή το ιχθυοπωλείο, δεν ξέρω τι να διαλέξω για το βραδινό.

Because this sentence describes a habitual or repeated situation: Whenever I pass by the butcher’s or the fish shop...

In Greek, όταν + present is very commonly used for things that happen whenever or every time in a general sense.

So:

• Όταν περνάω... = When/Whenever I pass by...

It does not mean only one specific future moment here. It describes something that regularly happens.

Περνάω από means I pass by or I go past a place.

So:

• περνάω από το κρεοπωλείο = I pass by the butcher’s
• περνάω από το ιχθυοπωλείο = I pass by the fish shop

This is a very common Greek pattern:

• περνάω από + place

The preposition από often means from, but with verbs like περνάω, it can mean by/past.

They mean the same thing. Περνάω and περνώ are two forms of the same verb.

• περνάω is very common in everyday speech
• περνώ is also correct and common, sometimes felt to be a bit more concise or slightly more formal

So in this sentence, you could also hear:

• Όταν περνώ από το κρεοπωλείο...

But περνάω sounds completely natural.

They are in the accusative singular, because in Modern Greek, the preposition από takes the accusative.

So:

• από το κρεοπωλείο
• από το ιχθυοπωλείο

Both nouns are neuter singular, so the article is το.

You also see the accusative later:

• για το βραδινό

because για also takes the accusative.

Greek often repeats the article when listing or contrasting two nouns, especially when each one is being treated as a separate option.

So:

• το κρεοπωλείο ή το ιχθυοπωλείο = the butcher’s or the fish shop

Repeating the article sounds natural and clear.

English often avoids repetition more easily, but Greek is very comfortable with it.

Both words contain the ending -πωλείο, which means shop/store where something is sold.

So:

• κρεο- relates to meat
• κρεοπωλείο = butcher’s / meat shop

and

• ιχθυο- relates to fish
• ιχθυοπωλείο = fish shop / fishmonger’s

This ending appears in other words too, and it is useful for building vocabulary.

Because δεν is used to negate verbs in the indicative, that is, ordinary statements of fact.

So:

• ξέρω = I know
• δεν ξέρω = I don’t know

By contrast, μη(ν) is used mainly with:

• the subjunctive
• commands/prohibitions
• certain fixed expressions

That is why the sentence has:

• δεν ξέρω
  but
• τι να διαλέξω

not τι μην διαλέξω

This is a very common Greek structure.

• τι = what
• να διαλέξω = to choose / should choose

So τι να διαλέξω means something like:

• what to choose
• what I should choose

After verbs like ξέρω and δεν ξέρω, Greek often uses:

• question word + να + subjunctive

Examples:

• Δεν ξέρω τι να πω. = I don’t know what to say.
• Δεν ξέρω πού να πάω. = I don’t know where to go.

This is one of the most useful Greek patterns to learn.

Because after να, Greek normally uses the subjunctive, and here the form is the aorist subjunctive:

• να διαλέξω

The aorist subjunctive is often used for a single, complete action. In this sentence, the speaker means a single choice:

• I don’t know what to choose

That fits the aorist very well.

If you used the present subjunctive, it would suggest something more ongoing, repeated, or continuous, which is not the main idea here.

It means for dinner or more literally for the evening meal.

βραδινό is an adjective being used like a noun. The full idea is something like:

• το βραδινό φαγητό = the evening meal

But Greek often omits the noun when it is obvious, so:

• το βραδινό = dinner / the evening meal

Therefore:

• τι να διαλέξω για το βραδινό = what to choose for dinner

Yes. Greek word order is more flexible than English word order.

This sentence starts with the όταν clause:

• Όταν περνάω από το κρεοπωλείο ή το ιχθυοπωλείο, δεν ξέρω τι να διαλέξω για το βραδινό.

But Greek could also rearrange things for emphasis, for example:

• Δεν ξέρω τι να διαλέξω για το βραδινό όταν περνάω από το κρεοπωλείο ή το ιχθυοπωλείο.

The original version sounds very natural because it sets up the situation first, then gives the result.

Because the sentence begins with a subordinate clause:

• Όταν περνάω από το κρεοπωλείο ή το ιχθυοπωλείο

and then moves to the main clause:

• δεν ξέρω τι να διαλέξω για το βραδινό

Greek punctuation often uses a comma in this kind of structure, especially when the subordinate clause comes first. It works very much like English here.