Learn Polish: Trzeba czytać ten podręcznik cały dzień. - One must read this textbook all day.

In Polish, you can express a continuous period of time (how long an action takes) by putting the time phrase directly into the accusative case, without a preposition. This is called the bare accusative of continuous extent. It works perfectly here because the reading takes up the entire day.

Could I use the perfective verb przeczytać instead of czytać in this sentence?

No, because the phrase cały dzień emphasizes the ongoing duration and process of the action, not its completion. When you focus on how long an action takes (such as spending the whole day reading), you must use an imperfective verb like czytać.

Are both ten podręcznik and cały dzień in the accusative case here?

Yes, they are! Ten podręcznik is the direct object of the verb czytać, and cały dzień is the bare accusative of time. Because both the words for textbook and day are masculine inanimate nouns in Polish, their accusative forms look exactly the same as their nominative (dictionary) forms.

Since both noun phrases are in the accusative, how do we know which one is the object and which is the time phrase?

Polish relies on context, logic, and standard word order to make the meaning clear. Since a textbook cannot read, ten podręcznik has to be the object receiving the action. Time phrases like cały dzień are naturally understood as adverbs of duration, especially when placed at the end of the clause.