Learn Hardcore Czech: Moje ruka je unavená, protože celý den píšu na počítači. - My hand is tired because I write at the computer all day.

In everyday speech, if someone says bolí mě ruka or moje ruka je unavená, it usually means the whole arm (or at least from the wrist up), unless the context clearly suggests just the hand.

Why is the verb píšu and not something like psám? How does it come from psát?

The infinitive is psát (to write), but the present tense stem changes to píš- for most forms. It’s irregular.

Present tense of psát:

(já) píšu – I write / I am writing
(ty) píšeš – you write (sg)
(on/ona/ono) píše – he/she/it writes
(my) píšeme – we write
(vy) píšete – you write (pl/formal)
(oni) píšou / píší – they write

So píšu is the standard 1st person singular present form of psát. There is no form psám in Czech.

What’s the difference between píšu and píši?

Both píšu and píši are correct 1st person singular present forms of psát.

píšu – more colloquial, everyday, very common in speech and informal writing.
píši – more formal, bookish, often seen in literature, official documents, or very careful style.

In a normal sentence like this, celý den píšu na počítači sounds the most natural.

Why is it celý den and not something like celého dne?

celý den is in the accusative case, and here the accusative is used to express duration of time:

celý den – (for) the whole day
celý týden – (for) the whole week
celý rok – (for) the whole year

The form celého dne is genitive. You might see it in other structures, e.g.:

během celého dne – during the whole day
na konci celého dne – at the end of the whole day

But after a verb to show how long something lasts, Czech normally uses accusative:
píšu celý den, pracuji celý den, čekal jsem hodinu, etc.

Can I move celý den? For example, is Protože píšu celý den na počítači also correct?

Yes. Czech word order is freer than English. All of these are grammatically correct:

Protože celý den píšu na počítači.
Protože píšu celý den na počítači.
Protože píšu na počítači celý den.

The differences are mostly about emphasis and rhythm:

celý den píšu na počítači – slightly emphasizes how long you’ve been writing.
píšu celý den na počítači – more neutral.
píšu na počítači celý den – slightly emphasizes that it’s on the computer you’ve been writing all day (as opposed to, say, by hand).

In everyday speech, all three sound natural in the right context.

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

The choice of preposition and case changes the meaning:

na počítači (preposition na
- locative)
  – literally “on the computer”, but idiomatically “on the computer / using the computer”
  – focuses on the device as the platform/tool: píšu na počítači, hraju na počítači.
u počítače (preposition u
- genitive)
  – “at the computer”, physically sitting by the computer
  – focuses more on your location: sedím u počítače, jsem celý den u počítače.
na počítač (preposition na
- accusative)
  – usually implies movement toward the computer or aimed at the computer (e.g. něco dávám na počítač – I’m putting something onto the computer).
  – It would not be used for “writing on the computer”.

For “I’m writing on the computer”, the normal idiomatic choice is píšu na počítači.

How does počítač become počítači? What case is that?

počítač is a masculine inanimate noun. In the sentence, it appears as počítači because:

it follows na with the meaning “on” (location), and
after na in that meaning, Czech uses the locative case.

Declension (singular) of počítač (pattern like stroj):

Nominative: počítač – (ten) počítač
Genitive: počítače – bez počítače
Dative: počítači – k počítači
Accusative: počítač – vidím počítač
Locative: počítači – na/po/… počítači
Instrumental: počítačem – s počítačem

So na počítači = na + locative.

Why is there a comma before protože?

protože is a subordinating conjunction (“because”). It introduces a subordinate clause (protože celý den píšu na počítači) that gives the reason for the main clause (Moje ruka je unavená).

In Czech, you normally put a comma before such conjunctions:

protože – because
když – when/if
jestli – if
aby – so that / in order that
ačkoli – although

So the comma in Moje ruka je unavená, protože celý den píšu na počítači. is required by standard punctuation rules.

Why don’t we say Já píšu? Is it wrong to include já?

It’s not wrong to say Já píšu, but it’s usually not necessary.

In Czech, the verb ending already shows the subject:

píšu clearly means “I write / I am writing”.

Therefore, subject pronouns (já, ty, on, ona, etc.) are often dropped, unless you want to:

emphasize the subject:
- Já píšu na počítači, ne ty. – I’m the one writing on the computer, not you.
contrast different subjects:
- Já píšu na počítači a ona píše rukou.

In your sentence, there is no special emphasis, so píšu alone is the most natural.

Could we also say Moje ruka je unavená, protože jsem celý den psal na počítači? What’s the difference from píšu?

Yes, that sentence is correct too, but the time perspective is different.

Moje ruka je unavená, protože celý den píšu na počítači.
- píšu – present tense
- Suggests you are still writing at the moment of speaking (or it’s very closely connected to the present).
- Similar to English: My arm is tired because I’ve been writing on the computer all day.
Moje ruka je unavená, protože jsem celý den psal na počítači.
- jsem psal – past tense (I wrote / I was writing)
- Suggests that the writing happened earlier today and has now finished, but the result (tired arm) is still felt.

Both are perfectly fine; you choose depending on whether the activity is understood as still ongoing or already finished.

Could I say Má ruka je unavená instead of Moje ruka je unavená?

Yes, you can.

moje ruka and má ruka mean the same thing: my arm / my hand.
moje is the full form, neutral, very common in speech.
má is a shorter, slightly more formal or literary form, more frequent in writing and in fixed expressions.

In everyday spoken Czech, Moje ruka je unavená sounds more natural. In written or poetic style, Má ruka je unavená is also very good Czech.