Page 563 - Practical English Usage 3ed - Michael Swan, Oxford

We do not use so before a that-clause.
I hope that we'll have good weather. (NOT : h61'e 86, tittle It/e'U htltJe gtJ6tl
ItIetlt.ler.)
Note the special use of I thought so to mean 'my suspicions were correct'. Empty your pockets. Ah, I thought so! You've been stealing biscuits again.
So is not used after know (see 313).
You're late. - I know. OR I know that. (NOT I kMftI 86.)
2 negative structures
We can make these expressions negative in two ways. affirmative verb + not
Did you win?- I'm afraid not.
We won't be in time for the train. - No, I suppose not.
negative verb + so
You won't be here tomorrow?- I don't suppose so.
Will it rain? - I don't expect so.
Hope and be afraid are always used in the first structure.
I hope not. (NOT {ti6n't h61'e 36.)
Think is more common in the second structure.
I don't think so. (More common than I think not.)
3 so at the beginning of a clause
We can use so at the beginning of a clause with say, hear, understand, tell, believe and a number of other verbs. This structure is used to say how the speaker learnt something.
It's going to be a cold winter, or so the newspaper says. Mary's getting married. - Yes, so I heard.
The Professor'S iLL - So I understand.
For so after tell and say. see 540.
540 so with say and tell
1 instead of that-clauses
So can be used after say and tell instead of repeating information in a that-clause.
She's going to be the next president. Everybody says so. (= . . . Everybody says that she's going to be the next president.)
You've got to clean the car. - Who says so?
Taxes are going up. Bob told me so.
Note that so is used in this way mostly when we are talking about the authority for statements, about reasons why we should believe them. When we simply want to identify the speaker, we prefer that. Compare:
Jane's crazy. - Who says so?- Dr Bannister. Jane's crazy. - Who said that? - I did.
For so at the beginning of a clause (e.g. so tire newspaper says), see 539.3.
so with say and tell 540
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