a). A pat on the back: a word of praise, encouragement, or congratulations.
b). Pat on the back: to praise, encourage, or congratulate.
以上两个常用语的意思是一样的，只不过一个是名词，一个是动词，意思是向他人表示称赞，鼓励，或者祝贺。在日常生活中使用频率很高。
造句：
a). He has done a wonderful job on this project and he deserves a pat on the back.
b). Tristan’s mom patted him on the back for the cool sand castle he built.
会用了吧？现在去试试看吧to give someone a pat on the back，或者pat someone on the back。