时间状语前不用介词的几种情况
1. today,yesterday, tomorrow,tonight, yesterday/tomorrow morning(evening, afternoon),today week (下周的今天)
there is a football game tomorrow morning.
2. 表示时间的短语里面有next, last, one, this,these,those,every,each,some,any,all等单词，介词应该省略。
i will see you next week.
you can come any days you want. 你哪天来都可以
3. 在不定冠词 a/an (a/an的意思是每一....)频率短语中，不用介词。
three times a week 每周三次
forty hours a week 每周40小时
sixty miles an hour 每小时六十英里
4. 5.在in the same way, in this way, in another way等短语中，in常常被省略。
please go (in) this way.
这种就是所谓的名词作状语。那种理解都可以，只要认识到这种现象即可，什么名字不重要。