This touched Seryozha.

He felt he was not to blame for not having learned the lesson; however much he tried, he was utterly unable to do that.

As long as the teacher was explaining to him, he believed him and seemed to comprehend, but as soon as he was left alone, he was positively unable to recollect and to understand that the short and familiar word "suddenly" is an adverb of manner of action.

Still he was sorry that he had disappointed the teacher.

He chose a moment when the teacher was looking in silence at the book.