Page 561 - Practical English Usage 3ed - Michael Swan, Oxford

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so (conjunction) and then
So and then can both be used in replies, to mean 'since that is so', 'it follows from what you have said'.
It's more expensive to travel on Friday. '" Then/So I'll leave on Thursday.
I'll be needing the car. - Then/So I suppose I'll have to take a taxi.
So (but not then) can also be used when the same speaker connects two ideas, to mean 'it follows from what I have said'.
It's more expensive to travel on Friday, so I'll leave on Thursday. (NOT ••• Then :'llledrte tm Thttrstiay.)
For so used like and. see 157.14.
so (degree adverb): so tired, so fast meaning
So means 'that much' or 'to that extent'. It is used when we are talking about a high degree of some quality - in situations where very is also a suitable word.
I'm sorry you're so tired. (= I know you're very tired, and I'm sorry.)
It was so cold that we couldn't go out. (= It was very cold weather, and
because of that we couldn't go out.)
I wish she didn't drive so fast. so and vety
Very (see 611) is used when we are giving new information. So is mainly used to refer to information which has already been given, which is already known, or which is obvious. Compare:
- You're very late. (giving new information)
I'm sorry I'm so late. (referring to information which is already known)
- It was very warm in Scotland. (giving new information)
I didn't think it would be so warm. (referring to information which is already known)
emphatic use
In an informal style, so can also be used like very to give new information, when the speaker wishes to emphasise what is said. This structure is rather like an exclamation (see 195).
He's so bad-tempered! (= How bad-tempered he is!) You're so right!
so ... that
We use so, not very, before that-clauses.
It was so cold that we stopped playing. (NOT It ftItfS fie", eeltl that tee
~petl playillg.)
He spoke so fast that nobody could understand. (NOT He sPtJke Ite"YJYtst
thttr...) ~
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