Ja czytam książkę o jedzeniu.

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Breakdown of Ja czytam książkę o jedzeniu.

• czytać: to read
• książka: the book
• o: about

Questions & Answers about Ja czytam książkę o jedzeniu.

Why is Ja used at the beginning of the sentence? Is it mandatory?

Ja means I and is the first-person singular pronoun. In Polish you usually drop it because the verb ending already shows the subject. It’s included here for emphasis or clarity. You’d hear more naturally:
Czytam książkę o jedzeniu.

What is czytam and how is it formed?

Czytam is the first-person singular present tense form of the verb czytać (to read). The base infinitive is czytać, and you remove -ąć and add the present ending -am for “I”:
czytać → czyt + am = czytam

Why is książkę in that form and not książka?

Because książka (book) is feminine and it’s the direct object of the verb. Polish uses the accusative case for direct objects. The accusative singular of feminine nouns ending in -a changes to -ę, so książka becomes książkę.

What does the preposition o require, and why is jedzeniu used?