前面加be动词，例如：
1、He was studying the complex similarities and differences between humans and animals.他在研究人和动物之间错综复杂的相似与不同之处。
2、Linguists are studying the asymmetric use of Creole by parents and children.语言学家正在研究父母和孩子使用克里奥尔混合语时的不对等现象。
3、How long have you been studying English?你学英语多久了
