Learn Russian: В густом лесу есть старый пруд. - There is an old pond in the dense forest.

The noun лес (forest) is part of a special group of short masculine nouns that take a stressed -у ending in the Prepositional case when talking about physical location. If you were talking about a forest, you would use the regular ending (о лесе), but to say in the forest, you must use в лесу.

Since лесу ends in -у, why does the adjective густом not match that ending?

Even though лес uses a special location ending here, its grammatical gender is still masculine. Adjectives always take the standard case endings regardless of the noun's quirks, so густой becomes густом following the regular masculine Prepositional rules.

Why do we include есть in this sentence?

In Russian, to state that something simply exists in a specific location, you use есть. If we left it out and just said В густом лесу старый пруд, it would change the focus to the location of a known pond (The old pond is in the dense forest) rather than introducing a new one (There is an old pond).

Does the д at the end of пруд sound like an English D?