Сміття не лежить на кухні довго, бо я одразу кладу його в смітник.

They are related words, but they mean different things:

• сміття = trash / garbage / litter
• смітник = trash can / garbage bin / dumpster, depending on context

So in this sentence:

• Сміття is the thing being moved
• в смітник is the place it gets put into

In Ukrainian, сміття is normally a mass noun or collective noun, like trash or garbage in English. Even though it refers to a lot of little things, grammatically it is usually treated as singular.

That is why the verb is singular:

• сміття не лежить = the trash does not lie / stay

It is also why the later pronoun is singular:

• я кладу його = I put it

This is a very common Ukrainian pattern. Ukrainian often uses posture/location verbs where English would simply use be or stay.

• лежати = to lie
• стояти = to stand
• висіти = to hang

With objects, these verbs often describe how something is situated in space. So сміття лежить на кухні is a natural way to say that trash is sitting/lying around in the kitchen.

A natural English translation is often:

• The trash doesn’t stay in the kitchen long
• or The trash doesn’t sit in the kitchen long

rather than the more literal doesn’t lie in the kitchen long.

Because на кухні is the normal, idiomatic way to say in the kitchen in Ukrainian.

Even though на often means on, with some places it is used where English uses in or at. На кухні is one of those standard expressions.

So:

• на кухні = in the kitchen

You may sometimes see в кухні, but на кухні is the usual everyday choice here.

Кухні is in the locative case.

Here is the pattern:

• кухня = nominative singular
• на кухні = locative singular

The locative is used after на and в/у when they express location:

• на кухні = in the kitchen
• у кімнаті = in the room
• на столі = on the table

So the ending changes because the noun follows a preposition of location.

Here в смітник means into the trash can/bin, so it expresses movement toward/into a place. After в/у with motion, Ukrainian uses the accusative case.

So:

• в смітник = into the trash can → motion
• у смітнику = in the trash can → location

That is an important contrast:

• кладу в смітник = I put it into the bin
• лежить у смітнику = it is lying in the bin

Because Ukrainian third-person pronouns change form by case, and in the object form, його can refer to both masculine and neuter nouns.

For сміття:

• nominative: воно = it
• accusative object: його = it

So:

• Сміття ... я кладу його
  = The trash ... I put it

This feels odd to English speakers at first because його is often first learned as him / his, but in Ukrainian it can also mean it, depending on grammar and context.

Кладу is the 1st person singular present tense of класти:

• класти = to put, to place
• я кладу = I put / I am putting

In this sentence, it describes a habitual action or a general routine:

• бо я одразу кладу його в смітник
  = because I immediately put it in the trash can

So the speaker is not talking about one single future action, but about what they normally do.

This is about aspect.

• класти = imperfective
• покласти = perfective

In the present/future forms:

• кладу = I put / I am putting / I usually put
• покладу = I will put (once, as a completed action)

The sentence is explaining a general reason or habit, so the imperfective кладу is the right choice.

If you said покладу, it would sound more like a single future event:

• бо я одразу покладу його в смітник
  = because I’ll put it in the trash can right away

That does not fit the original sentence as well.

Одразу means immediately, right away, or at once.

In this sentence:

• я одразу кладу його в смітник
  = I immediately put it in the trash can

Its position is fairly flexible, but different positions can change emphasis slightly:

• я одразу кладу його в смітник
• я кладу його одразу в смітник
• я кладу його в смітник одразу

The original version sounds very natural and emphasizes the promptness of the action.

Both mean because.

• бо = because
• тому що = because

Бо is shorter and very common in everyday speech and neutral writing.
Тому що is a bit fuller and sometimes slightly more formal or more emphatic.

So this sentence could also be:

• Сміття не лежить на кухні довго, тому що я одразу кладу його в смітник.

But бо is perfectly natural here.

Because бо introduces a clause of reason, and in Ukrainian that clause is normally separated by a comma.

So the structure is:

• main clause: Сміття не лежить на кухні довго
• subordinate clause of reason: бо я одразу кладу його в смітник

That is why the comma is required.

No, Ukrainian word order is fairly flexible, but the original order is natural and neutral.

Original:

• Сміття не лежить на кухні довго, бо я одразу кладу його в смітник.

This starts with сміття, so the sentence is mainly about the trash and what happens to it.

You could rearrange parts, but the emphasis would change. For example:

• Бо я одразу кладу його в смітник, сміття на кухні довго не лежить.

This is still understandable, but it sounds more marked and less neutral. The original sentence is the most natural everyday version.