When do we really use each one of them?
It is a verb form and should be used as a verb in a sentence. Lost is its past tense and past participle.
I lost my wallet.
"Lose some of your weight", mom told her son.
This is a noun form and can be used as a subject, object, or a complement.
Loss of profit worries him no end.
During recession, businessmen incur substantial loss.
He was a great loss in the company.
Loose is an adjective and is used to describe something.
1. After a month's diet, his jeans became too loose for him .
2. His pet dog is on a loose. (break free)
The use of a word is based upon its role in a sentence-how it is used in context.
These three are just some of the words that are always used interchangeably without realising/knowing it.
The key is to identify the words' function in a sentence and then find out which word fits the role.