V sobotu ráno sedím dlouho v kuchyni a piji kávu.

Because in this sentence v sobotu is a time expression, meaning on Saturday.

• With days of the week, Czech often uses v + accusative:
  • v pondělí = on Monday
  • v úterý = on Tuesday
  • v sobotu = on Saturday

This is different from the more familiar use of v with the locative for location:

• v kuchyni = in the kitchen

So:

• v sobotu = time
• v kuchyni = place

That is why the same preposition v is followed by different cases here.

Ráno means in the morning / morning.

In v sobotu ráno, it combines with v sobotu to mean:

• on Saturday morning

It works as a time expression without needing a preposition here. Czech commonly stacks time expressions like this:

• v sobotu ráno = on Saturday morning
• v neděli večer = on Sunday evening
• v pátek odpoledne = on Friday afternoon

So ráno helps specify which part of Saturday.

Czech often omits subject pronouns when they are clear from the verb form.

Here:

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

The ending -ím already shows the subject is I, so já is not necessary.

You could say:

• Já sedím dlouho v kuchyni a piji kávu.

But that usually adds emphasis, something like:

• I am the one sitting in the kitchen...
• As for me, I sit...

In a neutral sentence, leaving out já is normal.

Czech does not form the present continuous the way English does.

In English, you say:

• I sit or I am sitting

In Czech, one present-tense verb often covers both ideas:

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

So Czech does not need a separate verb like am here.

The exact meaning depends on context:

• Teď sedím v kuchyni. = I am sitting in the kitchen now.
• Každé ráno sedím v kuchyni. = I sit in the kitchen every morning.

In your sentence, the context makes it sound like a scene happening at that time.

Yes, you can also say piju.

Both mean:

• I drink / I am drinking

The difference is mostly style:

• piji = more standard, written, careful
• piju = very common in everyday speech, more colloquial

So:

• V sobotu ráno sedím dlouho v kuchyni a piji kávu.
  sounds a bit more formal or book-like
• V sobotu ráno sedím dlouho v kuchyni a piju kávu.
  sounds very natural in conversation

A learner should recognize both.

Because kávu is the direct object of piji.

The verb pít takes the accusative case for what you drink:

• piji kávu = I drink coffee
• piji čaj = I drink tea
• piji vodu = I drink water

The basic dictionary form is:

• káva = coffee

But in the accusative singular it becomes:

• kávu

This is very typical for feminine nouns ending in -a:

• káva → kávu
• voda → vodu
• kniha → knihu

Because v with a static location usually requires the locative case.

Here the meaning is:

• in the kitchen

The noun is:

• kuchyně = kitchen

Its locative singular form is:

• kuchyni

So:

• v kuchyni = in the kitchen

Compare:

• v pokoji = in the room
• v domě = in the house
• v Praze = in Prague

This is different from v sobotu, where v is used in a time expression.

Here dlouho means:

• for a long time
• a long time

It is an adverb, modifying sedím.

So:

• sedím dlouho = I sit for a long time / I have been sitting a long time

It does not mean long as an adjective here.
For example:

• dlouhá kniha = a long book
• dlouho sedím = I sit for a long time

So the -o form tells you it is functioning adverbially.

Yes. Czech word order is fairly flexible, although different orders sound more or less natural depending on emphasis.

Your sentence:

• V sobotu ráno sedím dlouho v kuchyni a piji kávu.

is perfectly normal.

But you could also hear:

• V sobotu ráno dlouho sedím v kuchyni a piju kávu.
• Dlouho sedím v sobotu ráno v kuchyni a piju kávu.
• V kuchyni sedím v sobotu ráno dlouho a piju kávu.

These variations shift the emphasis a little:

• putting dlouho earlier emphasizes the duration
• putting v kuchyni earlier emphasizes the location

Czech often places old/background information earlier and important/new information later, but there is no single rigid word order like in English.

Yes. Both verbs are in the present tense and describe actions happening in the same situation.

• sedím = I am sitting
• piji = I am drinking

The conjunction a means and, so the sentence links two actions:

• I am sitting in the kitchen for a long time and drinking coffee

Czech does not need to repeat any subject pronoun, because both verb forms already show I.

Yes, sedět is imperfective, and pít is also imperfective.

Aspect matters because the sentence describes an ongoing or uncompleted situation:

• sedím = I am sitting
• piji = I am drinking

Imperfective verbs are the normal choice for:

• ongoing actions
• repeated actions
• general descriptions

A perfective verb in the present would usually refer to the future, not to what is happening right now.

So the imperfective forms here fit the meaning very well.

It can be understood either way, depending on context.

It may describe:

• one particular Saturday morning
• or a typical Saturday morning

For example:

• as a scene in a story: On Saturday morning, I sit in the kitchen for a long time and drink coffee
• as a habit: On Saturday mornings, I sit in the kitchen for a long time and drink coffee

Czech often lets context decide this.
If the speaker wanted to make the habitual meaning very explicit, they might add something like:

• každou sobotu ráno = every Saturday morning

It is natural, but piji makes it sound slightly more formal or written than everyday speech.

A very conversational version would often be:

• V sobotu ráno sedím dlouho v kuchyni a piju kávu.

Everything else is completely normal. So the sentence is good Czech; it just leans a little toward standard written style because of piji.

It is pronounced roughly as pi-yi, with three written letters but two syllable-like parts:

• pi-ji

The j is pronounced like English y in yes.

So:

• piji sounds roughly like પી-yi / pee-yee in broad approximation

This is one reason many speakers prefer the colloquial piju in everyday speech—it feels smoother in conversation.