He gasped when he saw the size of the tooth.
1. to gasp at sth : to take a quick deep breath with your mouth open, especially because you are surprised or in pain. 喘氣，倒抽氣
2. to gasp (sth) out: to have difficulty breathing or speaking 透不過氣；氣喘籲籲地說
yawn ：verb, means to open your mouth and breath out deeply, due to tiredness or boredom: 打哈氣
They started to yawn after the third hour in the meeting.
辨析：
breath : to take air into your lungs and send it out again
sigh : to breath out slowly and make a noise that shows you are disappointed, tired, relieved etc
pant : to breath quickly and noisily through your mouth, because you have been using a lot of effort 氣喘籲籲
gasp : to breath very quickly and deeply and with difficulty 上氣不接下氣
snore : to breath noisily while you are sleeping
wheeze : to breath with difficulty, making a noise in your throat and chest, because you are ill. 呼哧呼哧地喘氣
His asthma was making him wheeze.
例句：
Ollie gasped with pain and slumped forward. 奧利疼的直抽氣，往前栽倒。
The audience gasped at the splendor of the costumes. 華麗的服裝令觀眾驚嘆。
Brendan climbed slowly, gasping for air/ breath . 布倫丹上氣不接下氣地緩慢攀爬。
He came to the surface of the water gasping for air . 他浮出水面急促的喘著氣。
They gasped in astonishment at the news. 他們聽到消息驚訝得倒抽壹口氣。
She gave a little gasp and clutch George’s hand. 她輕輕地倒吸壹口氣，抓住喬治的手。