He smiled from ear to ear when he received this birthday gift.

当他收到这生日礼物时笑得合不拢嘴。

当中的smile from ear to ear是指因开心而笑得合不拢嘴，也可用grin from ear to ear。

He grinned from ear to ear when his father gave him a new bike.

当他父亲送给他一辆新自行车时他笑得合不拢嘴。