Alone, lonely, lonesome & lone

Alone suggests that a person or thing is separate - there are no others around.

Lonely (and informal US lonesome) refers to unhappiness caused by being alone.

Compare:

I like to be alone for short periods.

But after a few days I start getting lonely/lonesome.

Alone can be emphasised by all.

After her husband died, she was all alone.

Alone is not used before a noun.
Lone and solitary can be used instead; lone is rather literary.

The only green thing was a lone/solitary pine tree.