in the case of 'so,' it is usually functioning as a pronoun, which is a word that replaces a noun. When used as a pronoun, 'so' refers to a person or thing that has already been identified, and it can be used as a substitute for a noun phrase (i.e. 'the man').
1. Can you hand me so?
2. He loves his mother so.
when used as a pronoun, the word 'so' should always be preceded by a noun phrase (e.g. 'the man'). Using 'so' as a pronoun is more informal than using a noun phrase, so its usage should be determined by the context and the audience.
an adverb is a word used to modify or describe a verb, an adjective, another adverb, or a phrase or clause. In the sentence 'He spoke so softly,' 'so' is an adverb modifying the verb 'spoke.' When used as an adverb, 'so' is usually used to describe the degree or extent of an action.
1. She ran so quickly that I couldn’t keep up.
2. He is so inclined to help others in their hour of need.
3. She had so much difficulty expressing her thoughts that she eventually gave up.
when an adjective is used to compare two nouns, an adverb is used to compare two verbs. Additionally, the adverb 'so' is rarely used without an accompanying adjective (e.g. 'so quickly'). It is also important to note that 'so' should never be used before an adverb (e.g. 'She ran so fastly').