Learn Hardcore Greek: Παλιά τα πιάτα πλένονταν στο χέρι, τώρα όμως πλένονται στο πλυντήριο πιάτων. - In the past the dishes were washed by hand, but now they are washed in the dishwasher.

As an adverb, Παλιά means in the past, back then, formerly.
So Παλιά τα πιάτα πλένονταν στο χέρι = In the old days the dishes were washed by hand.
As an adjective (feminine singular), παλιά would mean old and would stand before a feminine noun:
- μια παλιά τσάντα = an old bag

Because in your sentence Παλιά stands alone at the start and does not describe a noun, it can only be an adverb meaning formerly / in the past.

What tense and voice are πλένονταν and πλένονται, and how would you translate them?

Both forms come from the verb πλένω (I wash), in the middle/passive voice.

πλένονταν
- tense: imperfect
- voice: middle/passive
- person/number: 3rd person plural
- meaning: they were being washed / they used to be washed
πλένονται
- tense: present
- voice: middle/passive
- person/number: 3rd person plural
- meaning: they are washed / they are being washed

So the contrast is:

πλένονταν = were (habitually) washed in the past
πλένονται = are washed now

Why is the imperfect πλένονταν used instead of something like πλύθηκαν?

The imperfect πλένονταν is used because the sentence talks about a habitual, repeated action in the past, not a single completed event.

πλένονταν (imperfect) = used to be washed, were being washed (regularly)
πλύθηκαν (aorist passive) = were washed (once / in one completed event)

If you said:

Παλιά τα πιάτα πλύθηκαν στο χέρι.

it would sound like you are referring to one specific occasion in the past when the dishes were washed by hand, which does not match the intended meaning of back then, that’s how it was generally done.

So πλένονταν is correct because it matches the idea of a general, ongoing practice in the past.

Why is the passive voice (πλένονταν / πλένονται) used instead of an active form like πλέναμε / πλένουμε?

Greek, like English, often uses the passive when:

The action is more important than who does it.
The agent is obvious (people in general, we, you).

Your sentence:

Τα πιάτα πλένονταν / πλένονται
= The dishes were washed / are washed

focuses on the dishes and on how they are washed (by hand vs in the dishwasher), not on the people doing it.

You could also say it in the active voice:

Παλιά πλέναμε τα πιάτα στο χέρι, τώρα όμως τα πλένουμε στο πλυντήριο πιάτων.
= In the past we washed the dishes by hand, but now we wash them in the dishwasher.

Both are grammatically correct; the passive version just keeps attention on the plates and the change of method.

Do πλένονταν and πλένονται mean that the dishes wash themselves (reflexive), or is this just passive?

Here they are passive, not reflexive.

In Greek, the middle/passive voice forms (like πλένομαι, πλένονται) can express:

Reflexive meaning (washing yourself):
- Πλένομαι = I wash myself / I have a wash
Passive meaning (something is washed by someone/something else):
- Τα πιάτα πλένονται = The dishes are washed

In your sentence:

Τα πιάτα πλένονταν / πλένονται

clearly means The dishes were/are washed (by someone, e.g. by people or by the machine).
The plates are not doing the action; they receive it, so this is passive, not reflexive.

Why is it στο χέρι and not με το χέρι for “by hand”? Is there a difference?

Both στο χέρι and με το χέρι are understandable, but στο χέρι is the more idiomatic expression for by hand / not using a machine, especially with washing:

Πλένω τα πιάτα στο χέρι.
= I wash the dishes by hand.
Πλένω τα ρούχα στο χέρι.
= I wash the clothes by hand.

Literally, στο χέρι = in/on the hand, but in this fixed phrase it means manually, not in a machine.

Με το χέρι (= with the hand) tends to emphasize the instrument (the hand as a tool). It is not wrong, but in this kind of contrast (hand vs machine), natives normally say στο χέρι.

Why do we say τα πιάτα with the definite article, when in English we just say dishes (without “the”)?

Greek uses the definite article much more often than English, especially for generic statements about a whole class of things.

Τα πιάτα πλένονταν στο χέρι.
literally: The dishes were washed by hand.
but meaning: Dishes used to be washed by hand.

Other typical examples:

Τα παιδιά αγαπούν τα γλυκά.
= Children love sweets.
Οι σκύλοι είναι πιστοί.
= Dogs are faithful.

Using πιάτα without the article here (Παλιά πιάτα πλένονταν…) would sound odd or would suggest some undefined plates, not plates in general.

So in Greek, τα πιάτα naturally expresses dishes in general, where English drops the article.

What does όμως add after τώρα, and can it be moved or omitted?

όμως is a contrastive conjunction, similar to however / but / though.

τώρα όμως πλένονται στο πλυντήριο πιάτων
= but now they are washed in the dishwasher / now, however, they are washed in the dishwasher

It highlights the contrast between past and present.

About position and omission:

You can omit it:
- …, τώρα πλένονται στο πλυντήριο πιάτων.
  Meaning is still clear, but the contrast feels a bit weaker.
You can also move it:
- …, όμως τώρα πλένονται στο πλυντήριο πιάτων.
- …, τώρα πλένονται όμως στο πλυντήριο πιάτων. (less common, more spoken style)

The most natural versions are:

τώρα όμως πλένονται…
όμως τώρα πλένονται…

In all cases, όμως signals “on the other hand / however”.

What is going on in the phrase στο πλυντήριο πιάτων? Why is πιάτων in the genitive, and could we say something like πλυντήριο των πιάτων instead?

στο πλυντήριο πιάτων literally is in the washer of dishes and is the normal way to say in the dishwasher.

στο = σε
- το = in the / at the
πλυντήριο = washer / washing machine
πιάτων = of dishes (genitive plural of πιάτο)

Greek very often uses noun + genitive to form compound-like expressions:

πλυντήριο ρούχων = washing machine (for clothes)
καθαριστήριο χαλιών = rug cleaning shop
μηχανή καφέ = coffee machine

So πλυντήριο πιάτων is essentially a fixed term for dishwasher.

Alternatives:

πλυντήριο των πιάτων
Grammatically correct, but sounds more like the washer of the dishes (more literal, less like a standard object name).
πλυντήριο για πιάτα
Also understandable (washer for dishes), but again not the normal fixed term.

In everyday Greek, πλυντήριο πιάτων is the standard phrase.

Can we just say στο πλυντήριο without πιάτων?

Yes, often context makes it clear:

If you are talking about dishes, στο πλυντήριο will usually be understood as in the dishwasher.
If you are talking about clothes, στο πλυντήριο will be understood as in the washing machine.

To avoid ambiguity or when introducing the idea, people often say the full form:

πλυντήριο πιάτων = dishwasher
πλυντήριο ρούχων = washing machine (for clothes)

Could we change the word order, for example say Τα πιάτα παλιά πλένονταν στο χέρι? Does that change the meaning?

Greek word order is fairly flexible, and different orders usually change emphasis, not basic meaning.

Possible variants:

Παλιά τα πιάτα πλένονταν στο χέρι…
– Emphasis on Παλιά (the time frame, back then).
Τα πιάτα παλιά πλένονταν στο χέρι…
– Slightly more emphasis on τα πιάτα first, then you add παλιά as extra information.
Τα πιάτα πλένονταν παλιά στο χέρι…
– Focuses on the action (were washed), with παλιά attached to the verb phrase.

All of these are grammatically correct and would be understood the same way. The default, most neutral-sounding version for this kind of contrast is the original:

Παλιά τα πιάτα πλένονταν στο χέρι, τώρα όμως πλένονται στο πλυντήριο πιάτων.

Who is actually doing the washing in this sentence? There is no we or they mentioned.

The sentence uses the passive voice and leaves the agent unmentioned, just like in English:

Τα πιάτα πλένονταν στο χέρι.
= The dishes were washed by hand.

We understand that people were doing it, but Greek (and English) don’t have to say by people or by us.

So implicitly:

In the past: People / we washed the dishes by hand.
Now: The dishwasher washes them.

Greek often omits the agent in passive sentences when it is:

Obvious (people, we, you), or
Not important to the message.

Here, the point is the method of washing (by hand vs in the dishwasher), not who exactly does it.