They are used in different situations. 인생 is used mostly in sentimental situations or when you want to talk about generic statements about life. ex) Life is so short = 인생은 너무 짧다 생명 is used a lot in science, but it is also used when you refer to your life insurance (생명보험) 생활 refers to your daily life, lifestyle and how you live. 목숨 refers to human life. So, you would sometimes hear serious news saying "Mark가 목숨을 잃었습니다" = "Mark Lost his Life" meaning that Mark died. 평생 means "all your life". So you could say "나는 평생 일만 했다" = "I've worked all my life" Hope this helps :)