So As A Conjunction

So can mean ‘as well’ or ‘also’ in the structure so + auxiliary verb + subject. This structure is used to add something to the sentence before. Note that the auxiliary verb used with so is the same as the auxiliary verb used in the first sentence.

• James can sing really well, so can his sister. OR James can sing really well, and so can his sister.

This structure can also be used with be and have.

• I was upset, and so were the others.
• ‘I have lost his number.’ ‘So have I.’

If the first sentence doesn’t have an auxiliary verb, we use do / does / did with so.

• Peter works hard, and so does his brother.
• ‘He just wants to get some peace of mind.’ ‘So do I.’

So as a subordinating conjunction

So can be used to refer to the reason for something.

• He had not paid the rent, so his landlord asked him to vacate the room.
• It is raining again, so we will have to cancel the trip.

In a more formal style, we can express the same idea using as- and since-clauses.

• Since he had not paid the rent, the landlord asked him to vacate the room. OR As he had not paid the rent, the landlord asked him to vacate the room.
• As it is raining again, we will have to cancel the trip.

Note that as and since introduce the reason while so introduces the result.