W niedzielę leżę długo w łóżku i czytam książkę.

Because w requires a specific grammatical case, and niedzielę is that case form.

• The dictionary form is niedziela (Sunday – nominative).
• After w meaning on (a day), Polish uses the accusative case.
• The accusative singular of niedziela is niedzielę.

So the pattern is:

• niedziela → w niedzielę (on Sunday)

Niedzielę is in the accusative singular (feminine).

With expressions of time, Polish often uses:

• Accusative to say when? or for how long?
  
  • w niedzielę – on Sunday
  • co niedzielę – every Sunday
  • cały dzień – all day

So w + accusative here answers the question kiedy? (when?) → w niedzielę (on Sunday).

It can mean either, depending on context and intonation:

• One specific Sunday:
  W niedzielę leżę długo w łóżku – This coming Sunday I (plan to) lie in bed for a long time (often with extra context like w tę niedzielę – this Sunday).

• Habit / general rule (most typical here):
  W niedzielę leżę długo w łóżku… – On Sundays I lie in bed for a long time and read a book.

Polish often uses the singular day name (in accusative) for habits:

• W sobotę sprzątam – On Saturdays I clean.
• W poniedziałek pracuję z domu – On Mondays I work from home.

You can also use the plural w niedziele (on Sundays) if you really want to stress every Sunday, but the singular is already natural for a habitual action.

Polish is a pro‑drop language: the subject pronoun is usually omitted when the verb ending already shows the person.

• leżę – 1st person singular (I lie)
• czytam – 1st person singular (I read)

Because the endings -ę clearly indicate I, you normally don’t say ja unless:

• you want to emphasize contrast:
  Ja leżę, a on pracuje. – I lie in bed, and he works.
• you want to clarify who is doing the action in a more complex context.

So W niedzielę leżę… already means On Sunday I lie…

The preposition w can take different cases depending on meaning:

1. w + locative → where? (location, static)

   • w łóżku – in bed (where I am)
   • w domu – at home
   • w pracy – at work

2. w + accusative → where to? (movement into) or when? (time)

   • wchodzę w pokój – I walk into the room (movement)
   • w niedzielę – on Sunday (time)

In your sentence:

• w niedzielę – when? → accusative
• w łóżku – where? → locative (łóżko → w łóżku)

So the same preposition w can trigger different cases depending on the question it answers.

Both are grammatically correct, but the meaning is slightly different:

• w łóżku – literally in bed; normal phrase for being in bed (usually under the covers, resting, sleeping, being lazy).
• na łóżku – literally on the bed; more neutral about just being on the surface (sitting, lying, putting things on it).

In everyday Polish, w łóżku is the natural idiom for what you mean here: lying in bed for a long time.

Leżeć is the infinitive (the form you look up in a dictionary): to lie (in a horizontal position).

Leżę is the 1st person singular present tense:

• ja leżę – I lie / I am lying

Basic present tense conjugation of leżeć:

• ja leżę – I lie
• ty leżysz – you lie (sg., informal)
• on/ona/ono leży – he/she/it lies
• my leżymy – we lie
• wy leżycie – you (pl.) lie
• oni/one leżą – they lie

In the sentence, leżę is used because the subject is I.

In Polish, the present tense of imperfective verbs (like leżeć, czytać) can express:

1. Action happening now:
   Teraz leżę w łóżku. – I’m lying in bed now.

2. Regular / habitual action (as in your sentence):
   W niedzielę leżę długo w łóżku. – On Sundays I lie in bed for a long time.

Context decides which meaning is intended. Because you have W niedzielę, it naturally reads as a habit, not as right now.

Polish punctuation rules say:

• You do not put a comma before i when it connects two verbs that share the same subject in one clause.

Here:

• Subject (understood): ja (I)
• Verbs: leżę and czytam
• Connector: i (and)

So you write:

• leżę długo w łóżku i czytam książkę – no comma.

You would use a comma before i when it connects whole clauses with their own verbs and often their own subjects, for example:

• Leżę długo w łóżku, i on też długo leży.
  (I lie in bed for a long time, and he also lies for a long time.)

Książka is the nominative singular form (dictionary form) of a feminine noun: book.

In the sentence, książkę is the direct object of czytam (I read), so it must be in the accusative case:

• nominative: książka – a book (subject)
• accusative: książkę – a book (object)

Examples:

• Książka leży na stole. – The book lies on the table. (subject → nominative)
• Czytam książkę. – I’m reading a book. (object → accusative)

So książkę is simply książka changed into the accusative case.

Yes, that is grammatically correct and often natural.

• W niedzielę leżę długo w łóżku i czytam.
  → On Sundays I lie in bed for a long time and read.

In Polish, if the object is obvious or not important, you can drop it. Here:

• The focus can be on the activity in general: I read (something).
• If you add książkę, you make it slightly more specific: I read a book (rather than, say, e‑mails or news).

So both versions are fine, depending on how specific you want to be.

The meaning is the same (I lie in bed for a long time), but there is a slight difference in emphasis / rhythm.

• Leżę długo w łóżku – neutral word order, very natural.
• Długo leżę w łóżku – puts a bit more emphasis on długo (for a long time), like: I (really) lie in bed for a long time.

Polish word order is relatively flexible, especially with adverbs like długo, but the version in your sentence (leżę długo w łóżku) is the most typical.

You have a few natural options. Two very common ones:

1. Future with “to be” + infinitive (neutral, planned future):

   • W tę niedzielę będę długo leżeć w łóżku i czytać książkę.
     (This Sunday I will be lying in bed for a long time and reading a book.)

2. Future with perfective verbs (focus on completed/limited actions):

   • W tę niedzielę długo poleżę w łóżku i poczytam książkę.
     – poleżę / poczytam suggest I will lie / read for a while (and that’s the plan or I’ll have the opportunity to).

Your original sentence with present tense:

• W niedzielę leżę długo w łóżku i czytam książkę.

normally reads as a habit, not a one‑time future plan, unless the context clearly makes it about the future.

The letter ę is a nasal vowel, but at the end of a word it is usually only slightly nasal or even almost like plain “e” in everyday speech.

• niedzielę → roughly: nye‑DZYE‑le (with a very light nasal quality on the last vowel; many speakers just say something very close to niedziele).
• książkę → roughly: KSHONSH‑ke
  
  • ą is nasal (like on in French bon / Polish wąż)
  • ę at the end again is often just a slightly nasal e.

Key point: Don’t over‑nasalize ę at the end of words. A soft e‑like sound is usually acceptable and sounds natural.