Co-verbs are a group of verbs that often act as prepositions in a clause with another verb, though they can act as verbs in their own right (prepositional constructions I, p. 155).
Amongst them are: -
Zai 在 ‘to be at, be in, at, in, on’
He is in China 他在中国
He studies at the Foreign Languages Institute 他在外语学院学习
Gei 给 ‘to, for (for indirect objects), to give to’
He gives me books 他给我书
He introduced me to his friend 他给我介绍他的朋友
Yong 用 ‘by means of, to use’
He is using a Chinese dictionary 他用汉语词典
Ti 替 ‘for, as a substitute for’
He is taking my place 他替我
I am writing a letter for my mother 我替我妈妈写信
Some co-verbs are almost always used in prepositional phrases, and only usually act as verbs in their own right with complements attached.
Dui 对 ‘Concerning, vis-a-vis, to be in front of , facing something/one’
He was facing the dormitory 他对着宿舍
He is very good to students 他对学生很好
Gen 跟 ‘in company with, to follow’
I follow my students to the Institute 我跟着我的学生
I am going to the shops with my students 我跟我的学生去商店
NB all co-verb constructions will come before the main verb in the way that prepositional or adverbial phrases come before the verbs they modify.