Meaning: a vulnerable spot or weakness. A small fault in a person or system that can result in its failure.

Example Sentences:
1.- The corrupt minister is regarded as the government’s Achilles heel and is expected to resign.
2.- Though he was a good person, his short temper was his Achilles heel.

Meaning: by one’s own choice. Voluntarily, willingly, without coercion.

Example Sentences:
1.- The managing director of the company didn’t have to be asked to resign – he went of his own accord.
2.- Do you think the officer gave that statement of his own accord or was he forced to do it by his superiors?

Meaning: another aspect of oneself, a very close friend or constant companion, a trusted adviser.

Example:
1.- Jack was the alter ego of Rose in the Titanic movie.
2.- Over the years the secretary has become the alter ego of his boss.

Meaning: to be pretentious/showy. Act proudly or arrogantly.

Example Sentences:
1.- Susan is a very smart girl but the fact that she gives herself airs about her job makes me like her a little less.
2.- You should stop giving yourself airs, I know that you have been promoted but you are still just one of us.