Although he couldn't understand a word they said, some things were clear enough.

And they never understand a word I say, of course.

I can't understand a word of what you are saying.

I couldn't understand a word these people said to me.

And I couldn't understand a word of what anyone was saying.

I couldn't understand a word, and they tried their best to make sure no one heard them.

He could not understand a word they said to him.

They are talking, but I do not understand their words.

She could not understand his words, or why he would say these things.

One of them said something, but she couldn't understand his words.