答：这是一个祈使句.Note是动词原形,意为请注意
that-从句是其宾语.
从句分析：
the time-- 主语
spent not speaking--定语,相当于：which is spent not speaking,为 which one speaks not speaking 之被动结构.句型：spend time (in) doing sth.之否定：spend time not doing sth.
can be--谓语（系动词之情态式）
meaningful--表语.
译：请注意,用在不讲话的时间也可以是有意义的.（讲话之间的不讲话时段）