Learn Russian: В лесу было так темно, что турист совсем растерялся. - It was so dark in the forest that the tourist got completely confused.

We use так because it modifies the adverb темно. You use так with adverbs and verbs. If you wanted to say "such a dark forest" (modifying a noun), you would use the adjective form такой instead: такой тёмный лес.

Why does лес end in -у (в лесу) instead of the usual prepositional -е?

лес (forest) belongs to a small group of short, masculine nouns that take a special locative ending -у when they answer the question "where?" (location). Other common examples are в аэропорту (in the airport) and на полу (on the floor).

Is было темно an impersonal expression?

Yes! When describing the environment, weather, or a general state, Russian uses the neuter past tense verb было plus an adverb (like темно). There is no subject like "it" in the Russian sentence.

How does the word растерялся relate to getting confused?

It comes from the verb терять (to lose). The prefix рас- often suggests scattering or things coming apart, and the reflexive suffix -ся makes the action affect the subject. So, растерялся literally implies that someone "lost themselves" or lost their bearings, leading to the meaning of getting totally confused, flustered, or bewildered.