The second most common word in English is ‘AND’ which is usually a conjunction. It’s also used in complex adverbial phrases:
and so on (A2 in the English Vocabulary Profile EVP)
Also, note that ‘and so forth‘ is not listed in the EVP, but is listed in OXFORD at B1:
TIME AND AGAIN (unlisted in the EVP)
BY AND LARGE (C1)
BY AND BY (unlisted)
Once and for all (C2)
THROUGH THICK AND THIN (C2)
If you support or stay with someone through thick and thin, you always support or stay with them in easy and difficult situations.
BY FAR AND AWAY (unlisted)
‘and’ is also used in interjections:
low and behold (unlisted)
in locative adverbial phrases:
to and fro (unlisted)
and of course in proper names.