Using so as an adverb and a conjunction
So can be used in the following ways:
As an adverb
When used as an adverb, so is followed by an adjective or another adverb.
- Why are you so unkind?
- She was so angry.
- He was so ill that we had to hospitalize him.
So + auxiliary verb + subject
This structure is used in short answers.
- ‘I’m so hungry.’ ‘So am I.’ (Both speakers are hungry.)
- ‘I’ve a headache.’ ‘So have I.’ (Both speakers have a headache.)
Note that in this structure we use the same auxiliary verb in both clauses.
- Susie can sing well, and so can her sister. (Both Susie and her sister can sing well.)
If the first clause doesn’t have an auxiliary verb, we use do / does / did with so.
- ‘He just wants the best for his children.’ ‘So does every parent.’
So + adjective + a / an + singular countable noun
- I had never before seen so big a shark. (= I had never before seen such a big shark.)
- She was so gentle a woman that people loved and respected her. (= She was such a gentle woman that people loved and respected her.)
Note that only singular countable nouns are used in this structure.
So as a conjunction
As a conjunction, so is used to connect two clauses.
- She asked me to go, so I went. (= She asked me to go. Therefore, I went.)
- I was hungry, so I decided to eat something.
- She wanted to pass the test, so she worked hard.
- I was ill for several months so I lost my job.
Note that so introduces the result; as, since and because introduce the clause.
- As I was hungry, I decided to eat something.
- Since she didn’t work hard, she failed the test.
- Because I was ill for several months, I lost my job.
Clauses introduced by so cannot come at the beginning of a sentence. As-, since- and because-clauses can begin a sentence.