What part of speech is the word ‘SO’?

Although ‘so’ is hard to put a part of speech to, it is usually a general or degree adverb, but it can often also be a conjunction. Here is a ranked list in order of most frequent first with highlighted examples:

1.  general adverb – “So much for studying grammar everyday.” (RR)
2.   suborinating conjunction – “I need to pass, so I study often.” (CS)
3.  degree adverb – “Thanks so much for explaining this to me.” (RG)
4. Two part conjunction “‘Insofar as possible, English grammar pro is here to help.” (CS21)

Other:

8 SO (RR32) 350313

every so often

11 SO (BCL21) 117210

14 SO (RR22) 64795
15 SO (JJ21) 53934
16 SO (RR31) 38501

18 SO (RG22) 27383
(CS42) ‘In so far as‘) is a four part conjunction.