Learn to speak fluent English -- Accent Training, Correct Grammar usage, Idioms, Phrases and vocabulary with example sentences.

When do we really use each one of them?

LOSE(v) / LOST ( Past participle of Lose)

It is a verb form and should be used as a verb in a sentence. Lost is its past tense and past participle.

example:
I lost my wallet.
"Lose some of your weight", mom told her son.

LOSS(n)

This is a noun form and can be used as a subject, object, or a complement.

example.
Loss of profit worries him no end.
During recession, businessmen incur substantial loss.
He was a great loss in the company.

LOOSE(adj.)

Loose is an adjective and is used to describe something.

example:

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 realizing/knowing it.The key is to identify the words function in a sentence and then find out which word fits the role.