顺便问一句，你今晚有空吗？

By the way, are you free tonight?

Source: Tatoeba

Sentence Composition

1 顺便 shùnbiàn
  1. conveniently
  2. in passing
  3. without much extra effort
2 问 wèn to ask; to inquire
3 一句 yījù
  1. a line of verse
  2. a sentence
=4 你 nǐ you (informal, as opposed to courteous 您 nín)
=4 你 nǐ you (Note: In Taiwan, 妳 is used to address females, but in mainland China, it is not commonly used. Instead, 你 is used to address both males and females.)
5 今晚 jīnwǎn tonight
6 有空 yǒukòng to have time (to do sth)
=7 吗 má (coll.) what?
=7 吗 mǎ used in 嗎啡 mǎfēi
=7 吗 ma (question particle for "yes-no" questions)