Learn Turkish: Bütün gün çalıştım.

Bütün acts as an adjective meaning 'the whole' or 'entire' directly before a noun. Bütün gün means 'the whole day.' Hepsi is a pronoun meaning 'all of it' or 'all of them,' and it replaces a noun completely. You cannot put hepsi directly in front of a noun like gün.

Why is gün singular here? Doesn't 'all' usually imply a plural?

In this specific phrase, bütün means 'the entire' or 'the whole' of one specific thing. So bütün gün means one single 'whole day.' If you wanted to say 'all the days,' you would make the noun plural: bütün günler.

Could I use tüm instead of bütün in this sentence?

Yes! Tüm gün çalıştım means exactly the same thing. Bütün and tüm are often interchangeable in Turkish when they mean 'whole' or 'entire.'

Let's review the verb. Why does the past tense suffix on çalıştım start with a 't' instead of a 'd'?