Inasmuch as it pains me to say it, notwithstanding my affection for fused words, nevertheless, I encourage readers to use some of the words listed hereinafter sparingly and others not at all.
Evolution of the English language includes a process called univerbation (yes, that’s really a word), the combination of a fixed expression of two or more words into a single word. It’s how two nouns are transformed into a closed compound such as lightbulb, or how a phrase such as “forget me not” coalesces into a noun such as forget-me-not, to represent something new. It’s how phrases like “young urban professional” are abbreviated to words such as yuppie, and how a portmanteau word like smog is formed.
It’s how a verb phrase like “going to” is slurred to sound like gonna, and how a combination of a preposition and an infinitive such as in and to becomes the preposition into and how three words like not, with, and standing, which in sequence make little sense, are welded together to become a (highly formal) synonym for despite. (Other fused words are still considered invalid, such as the ubiquitous alot and alright and the rarer moreso and eachother, but in centuries to come these will likely be considered standard.)
It applies also to when two or three words (sometimes with the aid of another word that remains independent) unite to perform an adverbial function or to serve as a conjunction or as a pronoun. Note, however, that in many cases, these words—though no more venerable than any of the other English vocabulary that has survived for multiple centuries with little change—may be perceived as archaic or at least stuffy. When it comes to the words and phrases listed below, clear communication might be better served by employing a more transparent phrase that represents the same idea.
Many compound adverbs, such as those beginning with any (anyone, anything, and so on), are entirely acceptable, as is the slightly more formal however, as well as thereafter, therefore, nevertheless, and nonetheless, but the following words may be seen as pedantic:
hereinafter: following this part of this document or writing
hereinbefore: preceding this part of this document or writing
heretofore: up to this time
hitherto: up to this time
howsoever: in whatever manner, to whatever degree or extent
insomuch: to such a degree
therein: in that place, thing, or time, or in that particular or respect
thereinafter: following the part of that document or writing
thereinbefore: preceding the part of that document or writing
theretofore: up to that time
Some compound conjunctions (such as although) are familiar, but the following might be seen as distractingly formal:
albeit: even though
forasmuch as: in view of the fact that
howbeit: even though
inasmuch as: in view of the fact that, or in the degree that
insofar: to such degree or extent
whensoever: at any or every time
wheresoever: anywhere at all
Whatever, whenever, and so on are everyday words, and whatnot is common though it may be perceived as substandard dialect, but the following are stiff:
whatsoever: anything or everything, or no matter what, or anything that might also be mentioned (also an adjective)
whosoever: whatever person, or no matter who
In summary, with few exceptions (such as nevertheless), consider avoiding words with infixes (words inserted between others to form a single word)—though they are forgivable when used whimsically—and note that even some infix-free fused words (such as therein) may be considered overly formal.