The word ‘so’ can be used instead of a that-clause after the verbs say and tell.
- They are getting married. Everybody says so. (= Everybody says that they are getting married.)
Here we use the word ‘so’ to avoid the repetition of the clause ‘that they are getting married’.
- ‘You are going to cook dinner’ ‘Who says so?’ (= Who says that I am going to cook dinner?)
- The government is introducing new tax laws. Mark told me so. (= Mark told me that the government is introducing new tax laws.)
I told you so
The expression I told you so is used to mean ‘I warned you, but you didn’t listen to me.’
- ‘Mummy, I’ve failed my test.’ ‘I told you so. You shouldn’t have spent the whole time sitting in front of that TV.’
With this meaning, so is not normally used after other verbs of saying.
‘So’ can also be used after the verbs think, hope, believe, expect, suppose and guess.
- ‘Is Samuel here?’ ‘I think so.’ (= I think that Samuel is here.)
- ‘Do you think that we are going to win?’ ‘Yes, I hope so.’ (= I think that we are going to win.)
Note that ‘so’ cannot be used before a that-clause.
- I hope that we will win. (NOT I hope so that we will win.)
‘So’ cannot be used after ‘know’.
- ‘You are late’. ‘I know.’ or ‘I know it.’ (NOT I know so.)