用 i was +任何一个past participle都要加上by吗？

没有指出宾语:
I was cautioned not to ride fast.
我被警告不要骑得太快。
明确指出宾语:
I was cautioned not to ride fast by the police.
我被警察警告不要骑得太快。

be+过去分词:被动语态.
当有如上的宾语时应当有by

不一定是用BY

当你强调动作承受者时没有必要加BY。