N-TITLE 先生 Mr is used before a man's name when you are speaking or referring to him.
...Mr Grant. 格兰特先生
...Mr Bob Price. 鲍勃·普赖斯先生
...Mr and Mrs Daniels. 丹尼尔斯夫妇
N-VOC 先生(有时加于“总统”、“主席”等职衔前,作为称谓) Mr is sometimes used in front of words such as 'President' and 'Chairman' to address the man who holds the position mentioned.
Mr. President, you're aware of the system. 总统先生,你是清楚这种体制的。