Force a smile指的是在你不想微笑的情况下微笑：

例句：

Even though I didn't want to be there, I managed to force a smile for the cameras.

虽然我并不想在那儿，我还是为了镜头强迫自己露出笑容。

Smile from ear to ear

指的是很快乐很满足的大笑，有眉开眼笑的意思：

例句：

After hearing the results of his exam he walked around campus with a smile from ear to ear.

听到他的考试成绩后，他眉开眼笑地在校园里漫步。