a long face别理解为：某人有一张长脸，真实含义是“哭丧着脸、耷拉着脸”。

【例句】
He took one look at her long face and said ‘What’s wrong?’
他看了一眼她哭丧的脸，问“怎么了？

When he failed in the exam of English he pulled a long face.
他因英语考试不及格而闷闷不乐。