The word "海阔天空" is formed by combining two parallel phrases: "海阔", meaning "the sea is wide", and "天空", meaning "the sky is vast". Each phrase consists of a noun ("海" for sea, "天" for sky) followed by an adjective describing its expanse ("阔" for wide, "空" for empty or spacious). The juxtaposition of these phrases creates a compound idiom that emphasizes boundless scope through the imagery of natural elements.