Hello Ravi, You mean "When should I use 'let'?" ?! "Let" is used in different contexts: * It could mean ' to permit ' or 'allow': "She lets him roam around." "The teacher lets the students take a break." * It is used as an auxiliary to express a request, proposal, or command, or to convey a warning or threat . "Let's go" "Just let me see you doing that again." * Also (in mathematical or philosophical discourse) used as an auxiliary to express an assumption or hypothesis: " Let 'a' equal 'b' " * In an informal way it is used to mean 'utter': * In conjugations : - let alone : much less , not to mention " I can't remember him, let alone his brother." - let loose: set free and informally it means to make a sound or remark suddenly.