Když je noc, nechci jít sám po ulici.

Here když means when: When it is night... / When it’s nighttime...

In Czech, když often introduces a time clause, just like when in English. In some contexts it can also mean something closer to if, but in this sentence the time meaning is the natural one.

The comma is there because Czech normally separates this kind of subordinate clause:

• Když je noc, nechci jít sám po ulici.

You could also reverse the order:

• Nechci jít sám po ulici, když je noc.

That still means basically the same thing.

Czech usually does not use a dummy subject like English it in sentences such as:

• It is night
• It is cold
• It is late

So Czech simply says:

• je noc = it is night
• je zima = it is cold
• je pozdě = it is late

The verb je is the 3rd person singular of být (to be), but Czech does not need an explicit subject here.

This is a very common Czech pattern:

• chtít / nechtít + infinitive
• to want / not want + infinitive

So:

• chci jít = I want to go
• nechci jít = I don’t want to go

Here:

• nechci = I don’t want
• jít = to go

So the whole part means I don’t want to go.

This is about Czech verbal aspect and motion verbs.

• jít = to go in one concrete instance, one direction, one occasion
• chodit = to go / walk regularly, habitually, or back and forth

In this sentence, the speaker means a specific situation:

• I don’t want to go / walk alone down the street when it’s night.

So jít fits well.

If you used chodit, it would sound more like a general habit:

• Když je noc, nechci chodit sám po ulici.
• When it’s night, I don’t want to walk around the street alone / I don’t like going alone along the street at night.

Both can work, but jít sounds more like one act of going, while chodit sounds more general or repeated.

Sám means alone or by oneself.

In this sentence:

• jít sám = to go alone

But sám behaves like an adjective, so it changes according to gender and number:

• masculine speaker: sám
• feminine speaker: sama
• plural masculine personal: sami
• plural non-masculine personal: samy

So a woman would usually say:

• Když je noc, nechci jít sama po ulici.

That is a very common thing learners notice: English alone does not change, but Czech sám/sama/... does.

After the preposition po, Czech usually uses the locative case.

So:

• po ulici = along the street / on the street / through the street area, depending on context

Here it means something like:

• walking along the street
• walking down the street

The noun is:

• ulice = street

After po, it becomes:

• po ulici

So ulici is locative singular here.

This is a good phrase to remember as a chunk:

• jít po ulici = to walk along/down the street

Not exactly.

• na ulici usually means in the street / out on the street
• po ulici often suggests movement along the street

So:

• jsem na ulici = I am in the street / outside on the street
• jdu po ulici = I’m walking along the street

In real usage, these can overlap a little depending on context, but in this sentence po ulici is natural because the speaker is talking about going/walking.

Because Czech often omits subject pronouns when the verb already makes the person clear.

• nechci already means I don’t want
• so já is not necessary

You could say:

• Když je noc, já nechci jít sám po ulici.

But adding já usually gives extra emphasis, like:

• As for me, I don’t want to go alone down the street at night.

So the version without já is the normal neutral one.

Yes. Czech word order is more flexible than English, although the neutral version is often the best one to learn first.

Neutral:

• Když je noc, nechci jít sám po ulici.

You can also say:

• Nechci jít sám po ulici, když je noc.

That is still correct. The difference is mainly about emphasis and information flow.

Czech often puts known/background information first and newer or more emphasized information later. But for a learner, the safest approach is:

1. learn the neutral order first
2. notice alternative orders as stylistic variations

Noc literally means night.

So když je noc is literally when it is night. In natural English that may become:

• when it’s night
• at night
• when it’s dark

Depending on the translation shown to the learner, English may use dark because that is often what the speaker really means in context. But the Czech word itself is still night, not a separate word meaning darkness.

Yes. Even though the meaning is already known, this is a useful grammar point: Czech and English do not always line up word-for-word.

• Když je noc literally: When it is night
• natural English: At night / When it’s dark
• jít sám po ulici literally: to go alone along the street
• natural English: to walk alone down the street

So a natural English translation may sound less literal than the Czech structure. That is normal and does not mean the Czech grammar is unusual.