Ⅰ. afraid “害怕”是形容词，只能作表语，而不能作定语，后接of 短语或不定式，构成 be afraid of sb. 和 be afraid to do sth
① She is afraid of a snake. 她害怕蛇。
② The little girl is afraid to go out at night.
afraid +that clause “恐怕”， 是婉转拒绝别人的一种表达方式。 如：
① I’m afraid (that) I can’t go to the party. My brother is sick. 恐怕我不能去参加聚会了。我弟弟病了。
Ⅱ. fear “害怕”是动词，与be afraid 往往通用，但不如它常用(特别是在口语中)。如：
① We fear no difficulty.我们不怕困难。
② He feared to speak his mind.他不敢说出自己的想法。
③ Fearing that he would catch cold, I went out to see him.因为怕他会受凉，我走去看他。
Ⅲ. frightened adj “受惊吓的、害怕的”可做表语，也可作定语。如：
① She is too frightened to move.她太害怕了不能动弹。
② A frightened girl is crying. 一个受惊的女孩正在哭。