Learn Hardcore Polish: Czytając słownik wieczorem, zapisuję w zeszycie najważniejsze nowe słowa. - While reading a dictionary in the evening, I write down the most important new words in the notebook.

Function: it works like “while reading / when (I am) reading”, describing an action that happens at the same time as the main verb (zapisuję).

Who is doing the action in Czytając słownik wieczorem? Why is there no subject there?

The subject is understood, not expressed. In Polish, an adverbial participle like czytając must refer to the same subject as the main verb.

Main clause: zapisuję → subject = I
Participial phrase: Czytając słownik wieczorem → also I

So the full underlying meaning is:
(Kiedy ja) czytam słownik wieczorem, (ja) zapisuję w zeszycie…
(When I read the dictionary in the evening, I write down in a notebook…)

Polish grammar considers it a mistake if czytając refers to a different person than the subject of zapisuję.

Why is there a comma before zapisuję?

The comma separates the participial phrase from the main clause:

Czytając słownik wieczorem, → introductory participial phrase
zapisuję w zeszycie najważniejsze nowe słowa. → main clause

Rules:

An adverbial participle phrase (czytając…) is usually separated by a comma when it comes before or after the main clause.
You could also write the same meaning as a full clause:
- Kiedy czytam słownik wieczorem, zapisuję… (also with a comma)

So the comma is required here by standard punctuation rules.

What case is słownik in, and why doesn’t its form change from the dictionary form?

Słownik is in the accusative singular as the direct object of czytając (from czytać coś – to read something).

Nominative: słownik
Accusative: słownik

For masculine inanimate nouns like słownik, the nominative and accusative singular are identical in form, so you do not see a change even though the case is different.

Why is it wieczorem and not w wieczór or just wieczór?

Wieczorem is the instrumental singular of wieczór, used here as an adverbial expression of time:

wieczór – evening
wieczorem – in the evening / at night (in the sense of evening)

Polish often uses instrumental forms to say “in/at [time of day]”, e.g.:

rano (in the morning – historically a case form)
po południu (in the afternoon)
wieczorem (in the evening)
nocą (at night)

W wieczór is not idiomatic in this meaning. You normally say wieczorem to express “in the evening”.

What tense and aspect is zapisuję, and why that choice here?

Zapisuję is:

Present tense,
Imperfective aspect,
1st person singular: I write down / I am writing down.

The imperfective is used because the sentence describes a habitual, repeated action:

When I read the dictionary in the evening, I (usually / generally) write down the most important new words.

If you used the perfective zapiszę (I will write down), it would sound more like a single, completed future event:

Czytając słownik wieczorem, zapiszę w zeszycie… → While reading the dictionary in the evening, I will (once) write down…

Why is it w zeszycie, not w zeszyt? What case is zeszycie?

Zeszycie is locative singular, governed by the preposition w.

Preposition w:

w
- locative → in / inside / at (location)
  - w zeszycie – in a notebook
  - w domu – at home
w
- accusative → into (movement towards the inside)
  - w zeszyt would be understood as into the notebook, but this is not how you express writing into it in Polish.

For writing, Polish uses the “static location” pattern:

zapisuję (coś) w zeszycie – I write (something) in a notebook.

So w zeszycie with the locative is correct.

What case is najważniejsze nowe słowa, and how do the endings work?

Najważniejsze nowe słowa is in the accusative plural neuter, as the direct object of zapisuję.

Breakdown:

słowa – neuter plural (nominative = accusative) of słowo
Adjectives must agree in gender, number, and case:
- najważniejsze – most important (neuter plural, acc/nom)
- nowe – new (neuter plural, acc/nom)

So you have:

(Ja) zapisuję [co?] → najważniejsze nowe słowa
- case: accusative
- number: plural
- gender: neuter

Why are there two adjectives (najważniejsze and nowe) before słowa, and does their order matter?

Typical word order for multiple adjectives in Polish is:

More “evaluative” / subjective / grading adjectives (e.g. najważniejsze – most important),
More “descriptive” / factual adjectives (e.g. nowe – new),
Noun (słowa).

So najważniejsze nowe słowa follows the natural pattern: evaluation → description → noun.

You can change the order to nowe najważniejsze słowa, but:

najważniejsze nowe słowa suggests: from the new words, I note down the ones that are the most important.
nowe najważniejsze słowa would sound less natural and could slightly emphasize “the new ones which are the most important (as opposed to older important words)”.

The original order is the most idiomatic here.

Could we drop one of the adjectives and still have a correct sentence?

Yes. All of these are grammatically correct, just less specific:

…zapisuję w zeszycie najważniejsze słowa.
I write down the most important words.
(doesn’t explicitly say they’re new)
…zapisuję w zeszycie nowe słowa.
I write down new words.
(no mention of how important they are)
…zapisuję w zeszycie najważniejsze nowe słowa.
I write down the most important new words.
(both: they’re new, and they’re the most important among the new ones)

Can Czytając słownik wieczorem, zapisuję… be rephrased with a normal clause instead of a participle?

Yes. A very common and fully natural equivalent is:

Kiedy czytam słownik wieczorem, zapisuję w zeszycie najważniejsze nowe słowa.
(When I read the dictionary in the evening, I write down the most important new words in a notebook.)

You can also use:

Czytam słownik wieczorem i zapisuję w zeszycie…
(I read the dictionary in the evening and write down…)

The participle czytając is just a more compact way to express “while reading / when I read”.

How would the sentence change if the subject were we or he instead of I?

The participle czytając stays the same; only the main verb changes form.

We (1st person plural):
- Czytając słownik wieczorem, zapisujemy w zeszycie najważniejsze nowe słowa.
  (While reading the dictionary in the evening, we write down…)
He / she (3rd person singular):
- Czytając słownik wieczorem, zapisuje w zeszycie najważniejsze nowe słowa.
  (While reading the dictionary in the evening, he/she writes down…)

In all cases, czytając must refer to the same person as the subject of zapisuje / zapisujemy / zapisuję.