這裡專指的是that當連接詞使用時可以略掉的情況，尤其是在非正式的環境裡。
1. 非直接講述情況 (indirect speech):
1-a 在許多日常使用的描述性動詞之後的that在非正式情況下可以省略，比如say, think, suggest:
Jame said (that) he was feeling better.
I think (that) you should take this course.
這部份不難理解，因為是日常用法(所以是非正式)，然後又因為常常在用不會搞混，所以不需要特別用that來區分不同的子句。
1-b 在 某些特定動詞之後的that 不能省略，例如reply, telegraph, shout:
James replied that he was feeling better.
(Not ...
She shouted that she was busy.
(Not ...
另外一些補充。摘自 Merriam-Webster Dictionary of Usage:
這些動詞後頭較常省略that: believe, hope, say, think
和前者比較，這些較少省略that: assert, calculate, hold, intend
關於這部份，一些較少用到的動詞如果不確定要不要接that，一個較保險的作法是 -- 都加，不然就是靠查字典來確定。
2. 接在名詞之後的that通常也不省略:
I do not believe his claim that he is ill.
(比.... his claim he is ill 自然）
He disagreed with Copernicus' view that the earth went round the sun.
(Not ...
關於這部份，因為一串名詞下來很容易搞混，這時用that 就可以清楚隔開不同的子句
其它關於名詞子句的部份請看另一篇: “that”的省略(2)
3. 形容詞之後(after adjectives):
在日常普遍使用的說 法裡頭that可以省略:
I'm glad (that) you're all right.
It's funny (that) he hasn't written.
We were surprised (that) she came.
4. 連接詞(Conjunctions):
在非正式的 場合裡，一些兩個字的連接詞裡的that可以省略，比如so that, such...that, now that, providing that, provided that, supposing that, considering that, assuming that.
5. 關係子句(Relative structures):
在關係子句裡，當關係代名詞that是受詞時可以省略，同樣的，也是特別是在非正式的 場合裏頭:
Look! These are the people (that) we met in Brighton.
Do it the way (that) I showed you.
=======
英文部份以及主要觀念/架構出自 Practical English Usage, Oxford, 2nd Edition, 1995, by Michael Swan
中文部份除了翻譯之外，一些進一步的解說主要是用我自己的話寫出。中英文有矛盾的地方以英文原書內容為主。
http://dictionary.reference.com/browse/that?s=t
Usage note
https://ahdictionary.com/word/search.html?q=that
Usage Note
下一篇: “that" 的省略(2) -- 及物動詞之後的名詞子句
