Аквариумный нагреватель уже включили.

Аквариумный means aquarium as an adjective, literally aquarium-related.

It comes from the noun аквариум (aquarium) plus the adjective ending -ный. So:

• аквариум = aquarium
• аквариумный = aquarium / for an aquarium

In this sentence, аквариумный нагреватель means aquarium heater.

Russian often uses adjectives where English uses a noun modifier. Compare:

• aquarium heater in English
• аквариумный нагреватель in Russian

Because нагреватель is the main noun, and аквариумный describes what kind of heater it is.

So the structure is:

• аквариумный = adjective
• нагреватель = noun

They must agree in gender, number, and case. Here both are:

• masculine
• singular
• accusative form that looks the same as nominative

So аквариумный нагреватель is a matched adjective+noun phrase meaning the aquarium heater or an aquarium heater.

It is in the accusative case, because it is the direct object of включили (turned on / switched on).

However, for a masculine inanimate noun like нагреватель, the accusative singular looks exactly like the nominative singular. So:

• nominative: аквариумный нагреватель
• accusative: аквариумный нагреватель

That is why the form does not change.

You know mainly from the verb form and the overall sentence structure.

The verb is включили, which is past tense plural. If нагреватель were the subject, the verb would need to agree with it:

• нагреватель включил = the heater turned something on, which does not make sense here
• нагреватель включился = the heater turned on, using a different verb form

So in Аквариумный нагреватель уже включили, the natural interpretation is:

• someone / they turned on the aquarium heater already

That makes аквариумный нагреватель the object.

Включили is the past tense plural form of включить.

Russian often uses the 3rd person plural past with no expressed subject to mean they in an indefinite sense, or more naturally in English, someone or people.

So this sentence does not necessarily mean that several people turned it on. It often just means:

• They already turned on the aquarium heater
• or more naturally: The aquarium heater has already been turned on / Someone already turned on the aquarium heater

This is a very common Russian pattern for an indefinite human subject.

Russian often omits subjects when they are:

• obvious from context
• unknown
• unimportant
• deliberately left vague

Here the focus is on the fact that the heater is already on, not on who did it.

So включили without a stated subject works like:

• they turned on
• someone turned on
• it was turned on (depending on how you translate naturally)

English usually wants some subject, but Russian is more comfortable leaving it unstated.

Уже means already.

It shows that the action happened before now or sooner than expected / by this point.

So:

• Аквариумный нагреватель уже включили = The aquarium heater has already been turned on / Someone already turned on the aquarium heater

It often adds the idea this has been done now; you do not need to do it.

Yes, the word order can change, because Russian word order is relatively flexible.

In this sentence:

• Аквариумный нагреватель уже включили

the object comes first, which puts focus on the aquarium heater.

Other possible orders include:

• Уже включили аквариумный нагреватель
• Аквариумный нагреватель включили уже

These are all possible, but the emphasis changes a bit.

The original sentence sounds natural if the speaker is talking specifically about that heater, perhaps in contrast to something else.

Very roughly:

• Аквариумный нагреватель уже включили = As for the aquarium heater, it has already been turned on.
• Уже включили аквариумный нагреватель = They already turned on the aquarium heater.
• Аквариумный нагреватель включили уже = more marked; emphasizes already at the end

Because включили is from the perfective verb включить, which presents the action as completed.

Here the idea is that the heater was successfully switched on, as a finished action.

Compare:

• включили = turned on, switched on, did turn on
• включали = were turning on / used to turn on / turned on repeatedly, depending on context

With уже, the perfective включили is especially natural because it emphasizes that the action has already been completed.

Literally, it means someone turned on the heater or they turned on the heater.

But in context, it often implies the resulting state too: the heater is on now.

Still, Russian distinguishes between:

• Аквариумный нагреватель уже включили = someone has already turned on the aquarium heater
• Аквариумный нагреватель уже включён = the aquarium heater is already switched on

So your sentence focuses more on the action/event, while включён focuses more on the state/result.

Russian has no articles.

So аквариумный нагреватель can mean:

• the aquarium heater
• an aquarium heater

Which one is correct depends on the context.

In many real situations, English would probably use the here:

• The aquarium heater has already been turned on

because both speaker and listener likely know which heater is meant.

A helpful stressed version is:

аквариу́мный нагрева́тель уже́ включи́ли

Approximate pronunciation:

• аквариу́мный = ak-va-ree-OOM-nyy
• нагрева́тель = na-gree-VA-tyel
• уже́ = oo-ZHÉ
• включи́ли = vklyu-CHEE-lee

The first word can feel long, and включили begins with a consonant cluster that may be tricky at first.

Yes, absolutely, if the context already makes it clear which heater you mean.

• Нагреватель уже включили = The heater has already been turned on

Adding аквариумный just makes it more specific:

• Аквариумный нагреватель уже включили = The aquarium heater has already been turned on

So whether you include the adjective depends on how much detail you need.