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.