我听到他在喃喃自语。用英语怎么说？

我听到他在喃喃自语。

I could hear him mumbling to himself.

could

aux. “can”的过去式，能够，打算，用于假设语气的条件句，用于虚拟语气的结论句；

例句
The regulations were relaxed and the prisoners could get out and about a bit.
规定有所放宽，犯人可以出去短暂放风。

hear

vt.& vi. 听到，听见；vt. 听说，得知，听取，审理；vi. 听，听见；

例句
When I listen to her play I can hear a new Nigel Kennedy.
从她的演奏中我听出了新一代的奈杰尔·肯尼迪。

him

pron. (he的宾格)他；

例句
They said goodbye to him as he got aboard the train at Union Station.
当他在联合火车站登上火车之际，他们向他道别。

mumbling

v. 含糊地说某事，叽咕，咕哝( mumble的现在分词 )；

例句
It was embarrassing and sad to see this man reduced to a mumbling wreck.
看见这个人变得口齿不清、弱不禁风，真是让人既难堪又悲哀。

to

prep. 向，朝着，到，关于，属于；adv. 朝一个方向的，到某种状态，关闭；

例句
I suggest you leave it to an expert...
我建议你把它留给专家来解决。

himself

pron. （反身代词）他自己，（用以加强语气）他亲自，他本人；

例句
He pleaded guilty before a court martial to absenting himself without leave.
在军事法庭上他对自己擅离职守表示服罪。