Learn Hardcore Greek: Όταν βάλεις τη συσκευή στην πρίζα και υπάρχει ρεύμα, λειτουργεί καλά. - When you plug the device into the socket and there is electricity, it functions well.

Βάλεις is the aorist subjunctive, 2nd person singular of the verb βάζω (“to put”).
In Greek, after όταν (“when”) talking about a future / conditional event, you often use the subjunctive, not the present:

Όταν βάλεις τη συσκευή στην πρίζα…
= When you (will) plug the device in…

If you said Όταν βάζεις τη συσκευή…, it would sound more like “When you are (habitually) plugging the device…” and is less natural in this instructional/contextual sentence about what will happen.

Is βάλεις from the verb βάζω or from another verb like βάλω?

It comes from βάζω.
Greek verbs often have a different stem for the aorist:

Present: βάζω (I put)
Aorist subjunctive: βάλω (1st person), βάλεις (2nd person), etc.

So:

βάζεις = you put / you are putting (present)
βάλεις = you (will) put, you (should) put (subjunctive, often used after όταν, αν, να)

Why is it τη συσκευή and not just συσκευή?

Τη συσκευή is definite accusative singular feminine:

η συσκευή = the device (nominative)
τη συσκευή = the device (object of the verb “put”, so accusative)

Greek usually keeps the definite article where English often drops it.
English: “Plug the device into the socket” – the article is also natural here, and Greek mirrors that: βάζεις τη συσκευή…

What is happening in στην πρίζα? Why στην?

Στην is a contraction of:

σε (in, at, into) + την (the, feminine accusative)

So:

σε + την πρίζα → στην πρίζα

Literally: “into the socket”.
This contraction is extremely common: σε + τον → στον, σε + τους → στους, etc.

What exactly does πρίζα mean? Is it the plug or the wall socket?

Η πρίζα is the wall socket / power outlet.

η πρίζα = socket / outlet
το φις or το βύσμα = the plug (the part on the cable)

So βάζω τη συσκευή στην πρίζα is literally “I put the device in the socket” → “I plug the device in.”

Why is ρεύμα used here? Does it just mean “electricity”?

Yes, in this context το ρεύμα means electricity / electrical power.

υπάρχει ρεύμα = there is (electrical) power
δεν έχει ρεύμα = it has no power / there’s no electricity

The same word can also mean “current” (water current, air current, political current, etc.), but here it is clearly electric current / mains power.

Why is there no article before ρεύμα in υπάρχει ρεύμα?

With υπάρχει (“there is”) Greek often omits the article when talking about something in general / indefinite:

Υπάρχει ρεύμα. = There is (some) power / There is power.
Υπάρχει ένα πρόβλημα. = There is a problem.

If you said υπάρχει το ρεύμα, it would sound like you’re referring to some specific power that was already identified, which is not the idea here.

Why is υπάρχει in the present tense if we’re talking about a future condition?

Greek often uses the present tense to talk about general truths or future results in conditional/temporal sentences:

Όταν βάλεις τη συσκευή στην πρίζα και υπάρχει ρεύμα, λειτουργεί καλά.
Literally: “When you put the device in the socket and there is power, it works well.”

In English we might also keep the present (“when… there is power, it works well”) to express a general condition. Greek does the same: the present tense expresses a repeated / general rule, not only the present moment.

Why is the main verb λειτουργεί and not something like δουλεύει?

Both can be used for machines:

λειτουργεί = it operates, it functions
δουλεύει = it works

In technical or more formal language (manuals, instructions), λειτουργεί is often preferred because it sounds a bit more technical / neutral.
Everyday speech would more often use δουλεύει καλά (“it works well”), but λειτουργεί καλά is very natural in written or semi-formal instructions.

Why is λειτουργεί in the present and not θα λειτουργεί?

In Greek, present tense is commonly used in the result clause of a general condition with όταν + subjunctive:

Όταν βάλεις… λειτουργεί καλά.
= “When you plug it in, it works well.” (general rule)

If you say θα λειτουργεί, it can give a sense of future prediction or sometimes ongoing future (“it will be working”), which is not necessary here. The sentence is stating a general fact about how the device behaves, so present is ideal.

Could the word order be changed, for example: Λειτουργεί καλά όταν βάλεις τη συσκευή στην πρίζα και υπάρχει ρεύμα?

Yes, that is grammatically correct:

Λειτουργεί καλά όταν βάλεις τη συσκευή στην πρίζα και υπάρχει ρεύμα.

Greek word order is fairly flexible. The original order:

Όταν βάλεις… λειτουργεί καλά.

emphasizes the condition first. Putting λειτουργεί καλά at the beginning emphasizes the result first. Both are natural; the choice is stylistic.

What is the difference between όταν and αν in this type of sentence?

Both can introduce conditions, but there is a nuance:

Όταν = when(ever) → usually implies something expected / usual
- Όταν βάλεις τη συσκευή στην πρίζα… = When you (do) plug the device in…
Αν = if → more neutral or hypothetical
- Αν βάλεις τη συσκευή στην πρίζα… = If you plug the device in…

Here, όταν fits well because it sounds like a regular or normal scenario: “whenever you plug it in and there is power, it works well.”

Why is it καλά and not καλός or καλή?

Καλά here is an adverb, meaning “well”:

λειτουργεί καλά = it functions well

Forms:

καλός (masc.), καλή (fem.), καλό (neut.) are adjectives → they describe nouns:
- καλή συσκευή = a good device
καλά is the adverb → it describes the verb:
- λειτουργεί καλά = it works well

So καλά is correct because it modifies the verb λειτουργεί, not a noun.