Note that the time spent not speaking can be meaningful,too.

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）

以note为先行词的同位语从句that the time spent not speaking

can be情态动词，表肯定推测

that 引导的宾语从句是：the time can be meaningful ,too.
spent not speaking 等于 定从句which was spent (in) doing something,

这是Note引导的祈使句 that是同位语从句
从句中 主语 the time spent not speaking 是祈使句用倒装 正常是spend time (in) doing sth
谓语 can be
宾语 meaningful