3 sentences

1. 这首交响乐的主旋律非常优美。

   zhè shǒu jiāo​xiǎng​yuè de zhǔ​xuán​lǜ fēi​cháng yōu​měi.

   The main melody of this symphony is very beautiful.

   Source: Dong Chinese Dictionary

2. 和平与发展是当今时代的主旋律。

   hé​píng yǔ fā​zhǎn shì dāng​jīn shí​dài de zhǔ​xuán​lǜ.

   Peace and development are the main themes of our times.

   Source: Dong Chinese Dictionary

3. 这部主旋律电影在票房上取得了巨大的成功。

   zhè bù zhǔ​xuán​lǜ diàn​yǐng zài piào​fáng shàng qǔ​dé le jù​dà de chéng​gōng.

   This mainstream film was a huge box office success.

   Source: Dong Chinese Dictionary