can’t help doing 造句

can't help doing 造句
can't help doing 忍不住做某事 跟must be 和must have been造句?
比如 他昨晚听到这个笑话肯定忍不住笑了 用英语怎么翻译

I couldn't help smiling when I heard her singing.
It must be Mr. Y, who likes to help others.
That man must have been a soldier. 他肯定当过兵
I'm sure he could't help laughing when he heard this joke last night.
肯定忍不住只能换种说法