Learn Russian: Бывало, мы весь день проводили в лесу. - We used to spend the whole day in the forest.

Because of the word бывало, which marks the action as a repeated habit in the past. In Russian, repeated past actions almost always require the imperfective aspect, even if each individual occurrence lasted for a complete, stated duration.

What is the grammatical role of бывало in this sentence?

Here, бывало acts as an introductory word meaning 'it used to happen' or 'we used to.' It is a frozen past-tense neuter form of the iterative verb бывать. Because it functions as an introductory word setting the scene rather than the main verb, it is separated from the rest of the sentence by a comma.

Why is весь день used here without a preposition?

In Russian, to express how long an action takes (duration), you use the bare accusative case without any preposition. Since день is an inanimate masculine noun, its accusative form весь день looks exactly like the nominative.

Why is the ending in в лесу -у instead of the regular prepositional ending -е?

The word лес belongs to a small group of short, usually single-syllable masculine nouns that take a special locative ending -у (which is always stressed) when used with the prepositions в or на to show physical location.