Learn Hardcore Polish: Niski uczeń czyta książkę w bibliotece. - The short student is reading a book in the library.

niski is an adjective modifying uczeń (“student/pupil”), which is masculine singular. In Polish adjectives must agree in gender, number, and case with the noun they describe.

Masculine singular (nominative): niski
Feminine singular (nominative): niska
Neuter singular (nominative): niskie

What case is uczeń in, and why?

uczeń is in the nominative case, because it’s the subject of the sentence (the one doing the reading). The nominative is the “dictionary form” and is used for sentence subjects.

What part of speech is czyta, and what does its ending tell us?

czyta is a verb in the present tense, third person singular. The ending -a signals that “he/she/it reads” (on/ona/ono czyta). If you wanted “I read,” you’d say czytam, “you read” (singular informal) is czytasz, etc.

How do you pronounce czyta, especially the cz and y?

cz is like the “ch” in English “chocolate,” an affricate [t͡ʂ].
y is a close central unrounded vowel [ɨ], somewhat between “i” in “sit” and “u” in “put.”
So czyta sounds roughly like “CHIH-tah.”

What case is książkę, and why does it look different from książka?

książkę is the accusative singular of książka (“book”). The accusative case marks the direct object of a verb (what is being read).
Because książka is feminine and inanimate, its accusative form becomes książkę (the -a ending changes to -ę).

How do you pronounce the ą in książkę, and why does it change to ę in the ending?

The letter ą is a nasal vowel [ɔ̃], similar to French “on.” In the accusative singular of feminine nouns ending in -a, the ending changes from -a to -ę, which is a nasalized [ɛ̃]. So książka [kɕɔ̃ʂ.ka] becomes książkę [kɕɔ̃ʂ.kɛ̃].

Why is bibliotece used instead of biblioteka?

bibliotece is the locative case of biblioteka (“library”). The preposition w (“in”) requires the locative case to indicate location.

Nominative singular: biblioteka
Locative singular: bibliotece

Can we switch the word order, for example say Uczeń niski czyta…?

Polish is relatively flexible, but the neutral word order is Adjective–Noun–Verb–Object–Adverbial. Putting uczeń before niski (as in Uczeń niski czyta…) sounds odd and unnatural. You could emphasize “short” by fronting it (Niski uczeń…), but you wouldn’t normally say Uczeń niski… in neutral statements.

What’s the difference between uczeń and student, since both can mean “student”?

uczeń refers to a pupil in primary or secondary school.
student usually means a university student.
So here uczeń is appropriate for a younger learner in a school context.