The extremely beautiful mountain was within sight. It was straight ahead.
In the sample sentence, the pronoun “it” replaces the noun “mountain” and also eliminates all of the modifiers that go with “mountain” because you don't say extremely beautiful “it.” At first you will be replacing simple nouns with pronouns, so there won't be any complex or difficult modifiers. Just remember that when a noun has any modifiers, such as adjectives or articles, they will disappear when that noun becomes a pronoun.