Learn Hardcore Czech: Můj bratr často sedí v noci u počítače a je na internetu. - My brother often sits at the computer at night and is on the internet.

In this sentence, Můj bratr často sedí..., it modifies the verb sedí (sits). Czech word order is fairly flexible, so you can move často around for slightly different emphasis:

Můj bratr často sedí v noci u počítače... (neutral: he often sits at night at the computer)
Můj bratr v noci často sedí u počítače... (slight emphasis on at night as the time when this often happens)
Často můj bratr sedí v noci u počítače... (more emphasis on often; sounds more marked)

All of these are possible, but beginners should stick with something like Můj bratr často sedí... as the most natural neutral word order.

Why do we say v noci for “at night”? What case is that?

v noci literally means in the night, but in Czech it corresponds to English at night.

Grammar:

The noun noc (night) is feminine:
- Nominative singular: noc
- Locative singular: noci

The preposition v (in) when used with time expressions like parts of the day usually takes the locative case:

v noci – at night
v létě – in (the) summer
v zimě – in (the) winter
v neděli – on Sunday

So noc → noci because v + locative is used here.

Why is it u počítače and not u počítač or v počítači?

The preposition u means at / by / near and always takes the genitive case.

Nominative: počítač (computer)
Genitive singular: počítače

So:

u počítače = at the computer / by the computer

If you said u počítač, that would be wrong because počítač is still in nominative, not genitive.

v počítači would literally mean inside the computer, which is physically different and not what you want for “sitting at the computer”.

Why does počítač change to počítače in u počítače?

Because of case inflection.

počítač is a masculine inanimate noun. In the genitive singular, its ending is -e:

Nominative sg.: počítač – the computer (subject form)
Genitive sg.: počítače – of the computer / at the computer (after u)

The preposition u always requires genitive, so the noun must change form:
u + počítače (not u počítač).

What does sedí express here? Is it like English “sits” or “is sitting”? Is there any aspect difference?

sedí is the present tense of the imperfective verb sedět (to sit). Czech doesn’t have a special continuous form like “is sitting”; one present form covers both:

On sedí. – He sits / He is sitting.

In your sentence, context and the adverb často (often) make it habitual:

Můj bratr často sedí v noci u počítače...
= My brother often sits at the computer at night / is often sitting there at night.

You could use sedává (a frequentative form) to stress the habitual nature even more:

Můj bratr často sedává v noci u počítače...
(He tends to / regularly sits at the computer at night.)

But sedí is perfectly normal and common for a repeated habit.

Why is it je na internetu and not something like je internet or má internet?

je na internetu literally means is on the internet, and idiomatically it means is online / is using the internet.

je internet – would mean “there is an internet” or “he is an internet” (nonsense in this context).
má internet – means “he has internet” (he has an internet connection at home), not that he is currently online.

So:

je na internetu – he is on the internet (right now, or typically at that time).

Why do we say na internetu instead of v internetu?

In Czech, many abstract “spaces” take na + locative, even though English usually says “on” or “in”:

na internetu – on the internet
na telefonu – on the phone
na Facebooku – on Facebook
na univerzitě – at the university

So na internetu is the idiomatic, fixed expression for “on the internet”.
v internetu would sound strange; it’s grammatically possible but not used in everyday language for this meaning.

Why does internet change to internetu in na internetu?

Again, this is due to case.

internet is a masculine inanimate noun of a different declension type from počítač. Its locative singular is internetu:

Nominative sg.: internet
Locative sg.: internetu

The preposition na in the sense of on/at (a place or abstract space) usually takes the locative:

na internetu – on the internet
na stole – on the table
na koncertě – at a concert

So na + internetu is preposition + locative form.

Could the word order be Můj bratr sedí často v noci u počítače a je na internetu? Does it change the meaning?

Yes, Můj bratr sedí často v noci u počítače... is grammatically correct. Word order in Czech is flexible and is often used to express nuance or emphasis.

Rough differences in feel:

Můj bratr často sedí v noci u počítače...
Neutral, “often” is an unstressed adverb of frequency.
Můj bratr sedí často v noci u počítače...
Slightly more emphasis on the act of sitting and its frequency; it can sound a bit more marked or stylistically heavier.

For everyday neutral speech, Můj bratr často sedí v noci u počítače... is the most natural version.

Why is the conjunction a used here, and can we drop je and just say ...u počítače na internetu?

a is the basic conjunction meaning and, linking two actions:
1. sedí v noci u počítače – he sits at the computer at night
2. je na internetu – he is on the internet

So the sentence describes two related but separate facts about what he does at night.

You cannot simply drop je and say:

Můj bratr často sedí v noci u počítače na internetu.

That sounds like “sits at the computer on the internet” as one long noun phrase, and is ungrammatical / unclear in Czech. You need the verb je to form the second clause:

...u počítače a je na internetu.