Στη ντουζιέρα βάζω τη λεκάνη κάτω, για να μαζεύω το νερό όταν σαπουνίζω τον σκύλο.

στη is the contracted form of σε τη(ν).

So:

• σε = in / at / to
• τη(ν) = the feminine singular article in the accusative

In everyday Greek, σε + τη very often becomes στη.

Here στη ντουζιέρα means in the shower / in the shower stall area. Greek σε covers several ideas that English splits into in, at, and sometimes to; the exact meaning comes from context.

ντουζιέρα usually means the shower area, especially the shower tray / shower stall / shower base, not the showerhead itself.

So in this sentence, Στη ντουζιέρα refers to the place where the dog is being washed.

Because τη λεκάνη is the direct object of βάζω.

• βάζω = I put
• What do I put? τη λεκάνη = the basin / the tub

Also:

• λεκάνη is a feminine noun
• its singular accusative form is τη λεκάνη

That is why Greek uses the accusative here.

Here λεκάνη means a basin, bowl, tub, or container used to catch water.

It does not mean a toilet here. The word can have different meanings depending on context, but in this sentence it clearly means some kind of basin placed in the shower to collect water.

Literally, it means I put the basin down.

Here κάτω means down / low / on the floor below, so the whole phrase suggests:

• I put the basin down
• I place the basin on the floor / at the bottom

This is very natural Greek. It does not mean under in the sense of under something specific. If Greek wanted to say under something, it would often use κάτω από.

Greek word order is more flexible than English word order.

βάζω τη λεκάνη κάτω is a natural order:

• verb: βάζω
• object: τη λεκάνη
• adverb/location idea: κάτω

You could hear other orders in Greek too, depending on emphasis, but this version sounds normal and straightforward.

για να introduces purpose. It means:

• so that
• in order to
• so I can

So:

• για να μαζεύω το νερό = so that I can collect the water

This is one of the most common ways to express purpose in Modern Greek.

After για να, Greek uses a subjunctive-type construction, but in the present/imperfective, the verb often looks exactly like the ordinary present form.

So να μαζεύω looks like I collect / I am collecting, but here it means something like:

• so that I can be collecting
• so that I can collect regularly / continuously

The important point is that να changes the function of the verb, even when the form looks the same.

This is an aspect question.

• να μαζεύω = imperfective: ongoing, repeated, continuous
• να μαζέψω = perfective: one complete action

Here the speaker is talking about collecting water while washing the dog, probably as an ongoing or repeated process. So μαζεύω fits better.

It suggests something like:

• so that I keep collecting the water
• so that the water gets collected as I wash the dog

If the speaker said να μαζέψω, it would sound more like to collect it once / to gather it up as a complete action.

Yes, μαζεύω often means gather, collect, pick up, or bring together.

In this sentence, it means collect:

• μαζεύω το νερό = collect the water

So the idea is that the basin catches or gathers the water instead of letting it all run away.

Greek uses the definite article more often than English does.

Here το νερό means the water, but in natural English we might simply say water. Greek often uses the article when referring to something concrete and contextually understood.

So το νερό here means the specific water involved in the situation: the water in the shower.

όταν means when or whenever.

With σαπουνίζω in the present/imperfective form, the sentence gives a sense of:

• when / whenever I soap the dog
• while I’m soaping the dog

This fits a habitual or repeated situation very well.

So the speaker is not necessarily talking about one single event only; it sounds like something they do as a regular practice.

σαπουνίζω means to soap, to lather, or to apply soap to.

So:

• σαπουνίζω τον σκύλο = I soap the dog / I lather the dog with soap

It comes from σαπούνι, which means soap.

Because τον σκύλο is the direct object of σαπουνίζω.

• σαπουνίζω = I soap
• whom do I soap? τον σκύλο = the dog

Also note:

• nominative: ο σκύλος
• accusative: τον σκύλο

So the article changes to τον, and the noun loses the final -ς.

It sounds more like a habitual / repeated action.

Several things point in that direction:

• βάζω can describe a usual action: I put
• για να μαζεύω suggests ongoing or repeated collecting
• όταν σαπουνίζω τον σκύλο sounds like whenever / when I soap the dog

So the overall feel is:

• This is what I do when I wash the dog

rather than

• This is what I am doing right now only once