alone, lonely, lonesome and lone

Alone means ‘without others around’. Lonely (and informal AmE lonesome) means ‘alone and unhappy because of it’. 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 (see here). Lone and solitary can be used instead; lone is rather literary.

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