"笑口常开"用英语可以表达为"to have a constant smile on one's face." 这个短语源于中文，其中"笑"指的是笑容，"口常开"指的是常常露出笑容。一些英文例句及其中文翻译如下：

She always has a constant smile on her face. 她总是笑容满面。

He is known for his constant smile. 他以面常露笑容而闻名。

She is the kind of person who has a constant smile on her face, even in the most difficult situations. 她是那种即使在最困难的情况下也会面常露笑容的人。