Learn French: Il dort toute la journée parce qu'il est malade.

The word for day here is la journée, which is a feminine singular noun. The adjective tout must change to match the gender and number of the noun it describes. Because the noun is feminine and singular, we use toute.

Could I say tout le jour instead of toute la journée?

Not naturally. In French, journée is used to emphasize the duration of the day (from morning to night), which fits perfectly with a continuous action like sleeping. Jour is usually used as a simple unit of time, like when counting days.

Why is there an apostrophe in parce qu'il?

The phrase for because is parce que. However, since the next word (il) starts with a vowel, the e in que is dropped and replaced with an apostrophe to make it flow better when speaking. This is called elision.

I know the verb is dormir. Why is it spelled dort here?

Dormir (to sleep) is an irregular verb. To conjugate it in the present tense for il (he), elle (she), or on (one/we), you drop the -mir from the end and add a -t, giving you il dort.