xian sheng V.S lao shi

xiān shēng (先生)means Sir, Mister, or Husband.
examaple:
1. tā shì wǒ xiān shēng (他是我先生): He is my husband.
2. wáng xiān shēng ,nǐ hǎo (王先生,你好!)Hello, Mr.Wang.

lǎo shī(老师)is teacher. xiān shēng (先生)meant teacher in ancient time, but now we are not using it that often to call a person xiān shēng (先生)as a teacher!