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alone or lonely

Alone, can be used as an adjective or adverb. Either use means without other people or on your own.

For example: "He likes living alone."
"I think we're alone now." = There are just the two of us here.

Lonely is an adjective which means you are unhappy because you are not with other people.

For example: "The house feels lonely now that all the children have left home."

!Note - Just because you're alone, doesn't mean you're lonely.