他们住在一起的英语怎么说

"They live together" 英语可以说成 "They cohabit"，cohabit 来自拉丁语 "cohabitare"，意为 "共同居住"。

例句：

They have been cohabiting for over a year now. (他们已经共同居住了一年多了)
The couple decided to cohabit before getting married. (这对夫妇决定结婚前先共同居住)
It's more common for young people to cohabit before getting married these days. (现在年轻人结婚前共同居住更普遍了)