Když je venku zima, moje dcera spí pod teplou dekou.

Here když means when or whenever.

So Když je venku zima, ... means When it is cold outside, ... or Whenever it is cold outside, ....

A useful thing to know is that když can also sometimes mean if in everyday Czech, depending on context. In this sentence, though, when/whenever is the most natural reading.

Czech usually does not use a dummy subject like English it in weather or general-condition expressions.

English says:

• It is cold outside.

Czech says:

• Je venku zima.
• literally: Is outside cold.

So this is completely normal Czech. The verb je is there, but there is no separate subject word corresponding to English it.

In this sentence, zima means cold.

So:

• Je zima. = It is cold.

But zima can also mean winter, depending on context:

• V zimě = in winter
• Zima je dlouhá. = Winter is long.

So zima is one of those Czech words whose meaning depends on how it is used.

Venku means outside or outdoors.

So:

• Je venku zima. = It is cold outside.

It is an adverb, not a noun. You can think of it as giving the location of the condition.

You will also often see:

• ven = out
• venku = outside, outdoors

Because Když je venku zima is a subordinate clause introduced by když.

Czech normally puts a comma before or after subordinate clauses, just as English often does with an introductory clause:

• Když je venku zima, moje dcera spí pod teplou dekou.

This comma is required in standard Czech spelling.

Czech word order is more flexible than English, but this order is very natural.

• Když introduces the clause.
• je is the verb.
• venku gives the location.
• zima expresses the condition.

So Když je venku zima is a normal, idiomatic way to say When it is cold outside.

You may also hear slightly different word orders in Czech, because word order can shift for emphasis, but the version in your sentence is standard and neutral.

Spí is the 3rd person singular present tense of spát = to sleep.

So it can mean either:

• sleeps
• is sleeping

Which one sounds best depends on context. In this sentence, because the whole sentence describes a general situation, sleeps is probably the best English translation:

• When it is cold outside, my daughter sleeps under a warm blanket.

But grammatically, Czech present tense can also correspond to English is sleeping in the right context.

Moje dcera means my daughter.

Yes, moje can often be omitted in Czech if possession is obvious from context, especially with family members or body parts. So a Czech speaker might sometimes say simply:

• Dcera spí pod teplou dekou.

But including moje is also perfectly correct. It just makes the possession explicit.

Also, moje is a possessive pronoun agreeing with dcera, which is feminine singular.

Because the preposition pod can take different cases depending on meaning.

With location (where something is), pod takes the instrumental:

• pod dekou = under a blanket

With movement toward a place (where to), pod takes the accusative:

• pod deku = under the blanket / to under the blanket in the sense of moving there

In your sentence, the daughter is already sleeping there, so this is location, not movement. That is why Czech uses the instrumental:

• pod teplou dekou

Because both the adjective and the noun have to match the case, gender, and number required by the phrase.

The base forms are:

• teplá = warm
• deka = blanket

But after pod with a location meaning, Czech uses the instrumental singular.

For a feminine singular noun:

• deka → dekou

And the adjective agreeing with it also changes:

• teplá → teplou

So:

• pod teplou dekou = under a warm blanket

This is a very common Czech pattern:

• s dobrou kamarádkou = with a good female friend
• nad vysokou horou = above a high mountain

Literally, deka is blanket.

In real usage, though, exact bedding words do not always match perfectly between Czech and English. Depending on context, English might use:

• blanket
• cover
• sometimes even duvet in a looser translation

But the basic meaning of deka is definitely blanket.

Yes, you might hear that, and it would still be understandable. But Když je venku zima is more natural and idiomatic.

Czech often places venku before zima in this kind of expression:

• Je venku zima.

So for a learner, Když je venku zima is the safest version to remember.

Most naturally, it describes a general habit or usual situation:

• When it is cold outside, my daughter sleeps under a warm blanket.

That is because když plus present tense often expresses something habitual or generally true.

Of course, in the right context it could also refer to a specific situation, but without extra context it sounds like a general statement.

Then Czech would likely switch case after pod.

Compare:

• Moje dcera spí pod teplou dekou.
  = My daughter is sleeping under a warm blanket.
  → location, so instrumental

• Moje dcera si lehá pod teplou deku.
  = My daughter is lying down / getting under a warm blanket.
  → movement toward a position, so accusative

This location-vs-movement contrast is very important with Czech prepositions like pod, nad, před, za, and mezi.