The girl who lives next door is really pretty.

Literal

Next [at-に] live-ている girl [topic-は] very beautiful [copula-だ].

Relative clause 隣に住んでいる ('who lives next door') modifying 少女 ('girl'). とても ('very') + きれい ('pretty, clean, beautiful') + だ — basic descriptive sentence. きれい is a な-adjective.