感官动词变成被动语态后要加-to do或-doing

Answer:两句都不正确
错误的原因在于:当你把陈述句变为被动句时,宾语提前放到了句首,句子就不再有宾语了.如果还想表明谓语动作的施动者是谁,那就在句尾加上"by + 动作施动者(原来的主语)"
以上两句可改为:
I was seen to sing by him .
I was being seen singing by him.
(注:这里"singing"和"to sing "仍然是"I"的补语,只不过变成被动语态时,不定式的符号不能被省略)
被动语态的使用目的主要有以下三点:
(1)谁做的动作不知道
(2)说出谁做的没必要；
(3)动作承受者需强调
所以当把述句子变被动句时一般可以不再加上原来主语,如果还想表明谓语动作的施动者是谁,那就在句尾加上"by + 动作施动者"
If you still feel confused,leave a message please

要加to do
应改成I was seen to sing by him.

对呀，，但你这俩句子有点不合适，你的意思是他看见我唱歌（我唱歌被他看见），既然把“他him"放在了句中，那就不能省略by,所以应该是i was seen to sing by him,或者可以把by him都省去。 后面那个句子也是这样

是，两句都有毛病，主语不一致，且被看见是一瞬间的事，不能用进行时，was是多余的，要么I saw him singing,要么I was seen when he singing..

你的句子有问题，可改为：
I was seen to sing by him.他看见我唱了歌。（+to do强调做了某事）
I was being seen singing by him.我正在唱歌时被他看见了。（+doing强调正在做某事）