Lose: (Verb, present tense) have no longer, taken away from one by accident, carelessness, etc.
USAGE: The careless boy loses his pencil every day. Be careful, so that you don’t lose your money to thieves.
Lost: (Verb; past tense of lose) USAGE: The girl lost her book last week. The man lost two sons in the last civil war.
Loss: (Noun) act or loosing, failure to win or obtain.
USAGE: The business man suffered a heavy loss. The loss of so much money made him sick. The Bendel Insurance boys lost the match but their fans were not pleased about such a loss in their home ground.
Loose: (Adjective) free, not held or tied up, not tight, not strict, uncontrolled, immoral.
USAGE: The dog is too dangerous to be left loose. Jane is not the right girl for such a post, she is too loose in morals.
Loose: (Verb) Make free or loose. USAGE: Wine loosed his tongue and so he started to talk freely.
Loosen: (Verb) make or become loose. USAGE: My belt was too tight and so, I loosened it a bit. The screw has loosened.