Learn Hardcore Czech: Nezapomeň klíče, když jdeš z domu, a dobře zavři dveře. - Don't forget the keys when you leave the house, and close the door well.

zapomenout → zapomeň (imperative), with ne- added gives nezapomeň = don’t forget.
It’s addressed to one person you talk to as ty (informal singular “you”).

The perfective aspect fits because the speaker is talking about one concrete action in the future: at the moment of leaving, make sure you have not forgotten the keys.
An alternative imperative from the imperfective zapomínat would be Nezapomínej, which would sound more like “don’t (habitually) forget / stop forgetting” – focusing on repeated behaviour. Nezapomeň is the natural choice for a specific practical instruction here.

Could you also say Nezapomeň na klíče? What’s the difference between zapomenout něco and zapomenout na něco?

Yes, you could say Nezapomeň na klíče, and it would sound very natural.

Roughly:

zapomenout něco – “forget something” as a direct object, often a piece of information or the actual item:
- Zapomněl jsem heslo. – I forgot the password.
- Nezapomeň klíče. – Don’t forget the keys (don’t leave them behind).
zapomenout na něco – “forget about something / neglect something”, often an event, duty, or person, but also an item:
- Zapomněl jsem na schůzku. – I forgot about the meeting.
- Nezapomeň na klíče. – Don’t forget about the keys / Don’t forget to take the keys.

In everyday speech, both Nezapomeň klíče and Nezapomeň na klíče are acceptable; na klíče can sound a bit more like “don’t forget to take them into account / to take them with you”.

What case is klíče, and what is its singular form?

Klíče is the accusative plural of klíč (a key).

Singular:
- nominative: klíč
Plural:
- nominative & accusative: klíče

In the sentence, klíče is the direct object of Nezapomeň, so it must be in the accusative.

Why is there no word for you in the sentence?

Czech normally drops subject pronouns because the verb ending already shows the person and number.

Nezapomeň clearly marks 2nd person singular (ty), so ty is usually omitted.
If you add ty, it’s for emphasis: Ty nezapomeň klíče… – You don’t forget the keys (as opposed to someone else).

So the subject “you” is understood from the verb form.

What does když mean here, and why not až?

Here když means “when(ever)” in a general or habitual sense:

Nezapomeň klíče, když jdeš z domu…
≈ “Don’t forget the keys when (you) leave the house (in general, whenever that happens).”

Basic difference:

když – when(ever), as; neutral for past and present, and for general situations.
- Když jdu z domu, vždycky si beru klíče. – When I leave the house, I always take my keys.
až – when / once, especially for a specific future event.
- Až půjdeš z domu, nezapomeň klíče. – When you go / once you go out (this time), don’t forget the keys.

So with a general instruction or habit, když is more natural. For one particular future occasion, až (with future tense: půjdeš) is preferred.

Why is jdeš in the present tense, even though the instruction is about the future?

The clause když jdeš z domu expresses a general situation / habit, and Czech, like English, uses the present tense for that:

Czech: Nezapomeň klíče, když jdeš z domu.
English: Don’t forget your keys when you leave the house.

Both languages use the present tense for general “when” conditions, even if they’re about future occasions.

If you wanted to speak about one concrete future event, you would typically say:

Až půjdeš z domu, nezapomeň klíče. – When you (will) go out (that time), don’t forget the keys.

Why is it z domu and not z domů, z domova, or z baráku?

All of these exist but mean slightly different things:

z domu – from/out of the house (as a building).
- dům = a house, a building.
- z domu = from the house / out of the house.
  This is the most neutral choice here.
z domů – grammatically “from the houses” (plural).
- domy (houses) → genitive plural domů.
  You’d only use z domů if you literally meant multiple houses: Vrátili se z domů na kopci.
z domova – from home (more abstract, your home environment).
- domov = home (as a concept or background).
- Pracuju z domova. – I work from home.
  Nezapomeň klíče, když jdeš z domova is possible, but sounds more like “when you go away from home (in general)”, slightly more abstract than the specific building.
z baráku – from/out of the building / house, but colloquial.
- barák = informal for house / building.
- Nezapomeň klíče, když jdeš z baráku. – very informal speech.

So z domu is the standard, neutral variant: when you go out of the house.

What case is domu, and how is it linked to the preposition z?

Domu here is the genitive singular of dům.

The preposition z (“from, out of”) requires genitive:

z domu – from the house
z města – from the city
z práce – from work

So:

Nominative: dům (the house)
Genitive: domu → with z → z domu (“from the house”).

Why are there commas before když and before a in this sentence?

The sentence has two main clauses and one subordinate clause:

Main clause 1: Nezapomeň klíče
Subordinate clause: když jdeš z domu
Main clause 2: (a) dobře zavři dveře

Czech comma rules:

A subordinate clause introduced by když is always set off by commas:
- … klíče, když jdeš z domu, …
In this particular structure, the subordinate clause sits between two main clauses. The comma before “a” is actually the closing comma of the subordinate clause, not a comma directly “before and”:
- Nezapomeň klíče, [když jdeš z domu], a dobře zavři dveře.

If you removed the když-clause, you’d get:

Nezapomeň klíče a dobře zavři dveře. – no comma before a.

So both commas are there to mark the boundaries of když jdeš z domu.

What does dobře mean here, and could you replace it with something else or move it?

Dobře is an adverb meaning “well, properly”.

In dobře zavři dveře, it means:

“Close the door properly / tightly.”

Common alternatives:

pořádně zavři dveře – close the door properly / firmly.
důkladně zavři dveře – close the door thoroughly (a bit stronger, more formal).

Word order:

dobře zavři dveře – neutral, common.
Zavři dveře dobře. – also possible, but puts a bit more emphasis on dobře (on how you should close them), often sounding more emotional or contrastive.

In most everyday contexts, dobře zavři dveře or pořádně zavři dveře would be the most natural.

Why is zavři used and not zavírej? What’s the difference?

Zavři is the imperative of the perfective verb zavřít (“to close, to shut”).
Zavírej is the imperative of the imperfective verb zavírat.

Aspectual nuance:

zavřít → zavři – focus on a single, completed action with a result: “shut it (once, properly)”.
- Dobře zavři dveře. – Close the door properly (so that it ends up closed).
zavírat → zavírej – focus on a repeated or ongoing action: “(keep) closing / be closing”.
- Zavírej dveře potichu. – Close the door quietly (whenever you close it).
- Nezavírej dveře! – Don’t keep closing the door / stop closing the door.

For a one-time instruction like this, zavři is the natural choice.

Why is dveře plural, and what case is it in here?

Dveře (door) is a noun that is used only in the plural in modern Czech. Grammatically it behaves like plural feminine:

Nominative plural: dveře
Genitive plural: dveří
Dative plural: dveřím
Accusative plural: dveře
Locative plural: (o) dveřích
Instrumental plural: dveřmi

In dobře zavři dveře, dveře is the direct object of zavři, so it’s in the accusative plural.

It’s similar to English nouns like scissors or trousers, which are also grammatically plural even when referring to one item.

How would the sentence change if I wanted to speak politely or to more than one person (plural you)?

For polite or plural you, you use the 2nd person plural forms:

Nezapomeňte klíče, když jdete z domu, a dobře zavřete dveře.

Changes:

Nezapomeň → Nezapomeňte (imperative 2nd person plural)
jdeš → jdete (present 2nd person plural of jít)
zavři → zavřete (imperative 2nd person plural)

This can mean either:

polite you (addressing one person formally), or
addressing several people at once.

Can the word order be changed, for example by starting with Když jdeš z domu?

Yes, Czech word order is quite flexible, and several variants are natural:

Když jdeš z domu, nezapomeň klíče a dobře zavři dveře.
Very natural: starts with the condition, then the instructions.
Nezapomeň klíče a dobře zavři dveře, když jdeš z domu.
Also possible; the when-clause is at the end and sounds like an added condition (“…when you are leaving the house”).

The original:

Nezapomeň klíče, když jdeš z domu, a dobře zavři dveře.
inserts the když-clause in the middle. This is grammatically fine; stylistically, many speakers might prefer starting with Když jdeš z domu or putting the když-clause at the end, but all three orders are acceptable.