Learn Russian: От тоски он месяцами скитался в лесу. - Out of melancholy, he wandered in the forest for months.

In Russian, putting a time noun in the bare instrumental plural (like месяцами) expresses an extended, continuous duration or a recurring action. Here, it emphasizes that he spent whole months on end wandering. You will also see this pattern with words like часами (for hours) or годами (for years).

Why do we use the preposition от here to say "out of", rather than из or из-за?

The preposition от followed by the genitive case is commonly used to express an involuntary cause or a strong internal emotional state driving an action. Since тоска means melancholy, anguish, or severe boredom, от тоски perfectly captures that his internal feelings caused him to wander. You would not use из-за, which usually points to an external cause or obstacle.

What is the exact nuance of the verb скитался compared to other verbs of walking or roaming, like бродил?

While бродить means to wander or roam, often just for a stroll or while deep in thought, скитаться carries a heavier, more dramatic meaning. It implies wandering aimlessly over a long period, often being homeless, lost, or driven by hardship. It perfectly matches the dark mood of от тоски.

I learned that masculine nouns take -е in the prepositional case. Why does the sentence use в лесу instead of в лесе?