Learn Hardcore Polish: Nowy odkurzacz jest cichy, więc dzieci śpią, kiedy odkurzam pokój. - The new vacuum cleaner is quiet, so the children are sleeping when I vacuum the room.

It ends in a consonant → usually masculine.
In the accusative (object) it stays the same as nominative (odkurzacz → odkurzacz), which is typical for masculine inanimate nouns.

That’s why the adjective is nowy (masculine), not nowa or nowe.

Why is it jest cichy and not jest cicho or jest cichym?

These three forms have different functions:

jest cichy – adjective in nominative; describes a permanent/typical property:
- The new vacuum cleaner is quiet (as a characteristic).
jest cicho – adverb; describes the general sound level, not directly the vacuum:
- It is quiet (here / now).
  You might say: W pokoju jest cicho. – It is quiet in the room.
jest cichym odkurzaczem – adjective in instrumental after być, with a noun:
- He is a good teacher → On jest dobrym nauczycielem.
- It is a quiet vacuum cleaner → To jest cichy odkurzacz.
  (You normally wouldn’t say jest cichym alone.)

In your sentence, we have subject + być + adjective:
Nowy odkurzacz jest cichy. – The new vacuum cleaner is quiet.

What does więc mean, and how is it used?

więc is a conjunction meaning “so / therefore”.

Function:

It introduces a result/consequence clause.

In the sentence:

Nowy odkurzacz jest cichy, więc dzieci śpią...
– The new vacuum cleaner is quiet, so the children are sleeping...

Position:

Usually at the start of the clause it introduces:
..., więc dzieci śpią.
You generally don’t move it around inside the clause.

Why is there a comma before więc and before kiedy?

Polish is stricter with commas than English.

Comma before więc
Polish usually puts a comma before coordinating conjunctions such as a, ale, więc, lecz, i when they connect full clauses:
- Nowy odkurzacz jest cichy, więc dzieci śpią.
Comma before kiedy
Subordinate clauses introduced by words like kiedy, że, ponieważ, jeśli almost always have a comma:
- ..., kiedy odkurzam pokój.

So:
Nowy odkurzacz jest cichy, więc dzieci śpią, kiedy odkurzam pokój.

Why is it dzieci śpią and not something like dzieci śpiące?

dzieci śpią – normal present tense verb: the children are sleeping.
dzieci śpiące – a participle; would mean sleeping children as a noun phrase, not a full clause.

In Polish, when you want a full sentence “The children are sleeping”, you use a finite verb:

Dzieci śpią. – The children are sleeping.

You use the participle śpiące only to modify a noun:

śpiące dzieci – sleeping children
Widziałem śpiące dzieci. – I saw sleeping children.

What exactly is dzieci grammatically? Is it singular or plural, and what gender?

dzieci is:

The plural form of dziecko (child).
Grammatically behaves like non-masculine-personal plural.

Key points:

You say: dzieci są (are), dzieci śpią (sleep).
Adjectives referring to dzieci in plural take the non-masculine-personal form:
- małe dzieci – small children
- grzeczne dzieci – well‑behaved children

So: meaning “children”, grammatically “plural (non‑masculine‑personal)”.

Why don’t we say the pronoun ja in kiedy odkurzam pokój?

Polish is a pro‑drop language: subject pronouns are usually omitted when the verb ending shows the person.

odkurzam already tells you it’s 1st person singular (I).
- ja odkurzam
- ty odkurzasz
- on/ona odkurza
  etc.

So:

kiedy odkurzam pokój – when I vacuum the room
is more natural than kiedy ja odkurzam pokój, which is only used for emphasis (e.g. contrast: when I vacuum, not when someone else does).

What does odkurzam literally mean, and how is the verb formed?

odkurzam is the 1st person singular present tense of odkurzać – to vacuum, to hoover.

Morphology:

kurz – dust
prefix od- – here creates a verb meaning to remove dust
odkurzać – to vacuum (imperfective)
odkurzam – I vacuum / I am vacuuming

Conjugation (present):

ja odkurzam – I vacuum
ty odkurzasz – you vacuum
on/ona/ono odkurza – he/she/it vacuums
my odkurzamy – we vacuum
wy odkurzacie – you (pl.) vacuum
oni/one odkurzają – they vacuum

Why is it odkurzam pokój and not odkurzam w pokoju?

Because odkurzać takes a direct object in the accusative: you vacuum something.

odkurzam pokój – I vacuum the room (object = room)
odkurzam dywan – I vacuum the carpet

If you said odkurzam w pokoju, it would mean I vacuum (somewhere) in the room, without saying what you vacuum. That’s much less typical unless the object is obvious or previously mentioned.

Why is it pokój and not pokoju here?

pokój is a masculine inanimate noun.

Nominative singular: pokój
Accusative singular: pokój (same form as nominative for masculine inanimate)

In odkurzam pokój, pokój is a direct object, so it is accusative. The form happens to look the same as nominative.

You would use pokoju (genitive or locative) in other contexts:

Nie ma pokoju. – There is no room. (genitive)
W pokoju jest cicho. – It is quiet in the room. (locative)

Why do we use odkurzam (imperfective) and not odkurzę (perfective) here?

Aspect difference:

odkurzam – imperfective; focuses on the process / ongoing action.
odkurzę – perfective; focuses on the completion of the action (“I will have vacuumed”).

In the sentence:

... dzieci śpią, kiedy odkurzam pokój.
– The sleeping happens at the same time as the vacuuming; we care about the ongoing activity, not the completed result.

If you used odkurzę, it would sound more like a single finished event (and normally future):

Dzieci będą spać, kiedy odkurzę pokój. – The children will sleep when I (have) finished vacuuming the room.

Can I replace kiedy with gdy or jak? Is there any difference?

Yes, you can often replace kiedy with gdy or jak, with small stylistic nuances.

kiedy – neutral “when” in most contexts.
gdy – a bit more literary/formal, often interchangeable with kiedy.
jak – very common in spoken language with a similar meaning “when”.

In your sentence, all are possible:

..., kiedy odkurzam pokój.
..., gdy odkurzam pokój.
..., jak odkurzam pokój. (more colloquial)

Meaning is almost the same: they all introduce a time clause.

Can I change the word order of the whole sentence?

Yes, Polish word order is quite flexible. For example:

Dzieci śpią, kiedy odkurzam pokój, bo nowy odkurzacz jest cichy.
Kiedy odkurzam pokój, dzieci śpią, bo nowy odkurzacz jest cichy.
Bo nowy odkurzacz jest cichy, dzieci śpią, kiedy odkurzam pokój. (more marked/emphatic)

The basic neutral version is close to what you have:

Nowy odkurzacz jest cichy, więc dzieci śpią, kiedy odkurzam pokój.

Changes in order mainly affect emphasis and style, not core meaning.