Po práci sedím u stolu a piji čaj.

It is built into the verb forms.

• sedím = I sit / I am sitting
• piji = I drink / I am drinking

Czech usually drops subject pronouns when they are clear from the verb ending. You could say Já po práci sedím u stolu a piji čaj, but já would add emphasis or contrast, not just neutral information.

Because the preposition po takes the locative case when it means after in a time sense.

The noun práce changes like this:

• nominative: práce
• locative singular: práci

So:

• po práci = after work

This is a very common everyday phrase.

Usually it means after work in the general everyday sense: after your job, after you finish working, after the workday.

Czech has no articles, so the phrase itself does not tell you a/the. Context does that.

If you wanted to be more specific, Czech could make that clearer in other ways, for example:

• po té práci = after that work
• po dokončení práce = after finishing the work

But in normal conversation, po práci is the standard way to say after work.

Sedím is the 1st person singular present tense of sedět.

It means:

• I sit
• I am sitting

In Czech, the present tense often covers both the simple present and the present continuous that English separates.

Also, sedět is imperfective, so it describes a state or ongoing action, not the moment of sitting down.

• sedím = I am sitting / I sit
• sednu si = I sit down

Piji is the 1st person singular present tense of pít.

It means:

• I drink
• I am drinking

Yes, piju is also very common, especially in spoken Czech.

So:

• piji = standard, a bit more formal or careful
• piju = very common in everyday speech

Both are correct. A learner will often hear piju more often in conversation.

Because u stolu means at the table / by the table, which is the natural meaning here.

Compare:

• u stolu = at the table, by the table
• na stole = on the table

So:

• Sedím u stolu = I am sitting at the table
• Sedím na stole = I am sitting on the table

Those are very different.

Because u requires the genitive case.

The noun stůl changes like this:

• nominative: stůl
• genitive singular: stolu

So:

• u stolu = at/by the table

This is a normal case change. The noun does not always keep the same form after a preposition.

Čaj is the direct object of piji, so it is in the accusative case.

However, čaj is a masculine inanimate noun, and in the singular its accusative form is the same as its nominative form.

So:

• nominative: čaj
• accusative: čaj

That is why you do not see a visible change here.

Because Czech has no articles.

So čaj can mean:

• tea
• a tea
• the tea

The exact meaning depends on context.

This is one of the biggest differences from English. Czech speakers do not need a separate word like a or the in ordinary sentences.

It can be either, depending on context.

Because both verbs are in the present tense and are imperfective, the sentence can mean:

• a general habit or routine: After work, I sit at the table and drink tea
• something happening now, if the situation makes that clear: After work, I’m sitting at the table and drinking tea

Without more context, many learners would naturally read it as a routine or typical action.

Yes. Czech word order is more flexible than English word order.

The version here:

• Po práci sedím u stolu a piji čaj.

puts po práci first, which sets the scene: after work.

You can move things around for emphasis, but not every order sounds equally natural. Czech word order often reflects what is already known, what is being emphasized, and what sounds most natural in context.

So the sentence is not fixed in exactly the same way as English, but the given order is very normal.

Because a simply joins two coordinated parts here:

• sedím u stolu
• piji čaj

In a basic sentence like this, Czech normally does not use a comma before a, just like English normally does not use one before a simple and.

Yes, quite a lot.

The sentence uses imperfective verbs:

• sedím = I am sitting / I sit
• piji = I am drinking / I drink

These describe ongoing or habitual actions.

If you switch to perfective verbs, the meaning becomes more about completion or a single finished event. For example:

• Po práci si sednu ke stolu a vypiju čaj.

This means something like:

• After work, I sit down at the table and drink up a tea.

Now the focus is on:

• sednu si = I sit down
• vypiju = I drink it up / finish drinking it

So yes, aspect matters a lot in Czech.