Making up words requires suffixes—word endings. In English, there are two major types of suffixes. Some endings INFLECT nouns, verbs and adjectives, and adverbs to show their grammatical forms: plural, possessive, past tense, comparative, superlative. And some endings DERIVE new words from old, like –ify which makes a verb out of a noun or –ness which makes a noun out of an adjective or –y, which makes an adjective out of a noun (and which got quite a workout in the Buffy the Vampire Slayer TV series). As with prefixes, there are traffic rules. You can add –ness to clever, happy and sad but not intelligent, depressed and ecstatic. And we say curiousity or curiousness but verbosity rather than verboseness.
One suffix that’s puzzles me is –er. The vast majority of –er words are added to verbs to make them into one who ___s: teacher, farmer, professor (with the erudite variant –or). Lots of exceptions exist that involve implied or outright metaphor: lifer, looker, officer, lawyer, mother f***er. The –er words that are especially confusing to me are the words truther, birther, deather. And there’s a set of double –er compounds too: fixer upper, picker upper (check out early November for my attempt to add to this with breaker downer). I should have tried looker overer for the triple –er hat trick.
Some languages, by the way, use circumfixes and infixes too. English has a causative circumfixed em—–en, as in embolden or Homer Simpson’s embiggen. But we don’t use that technique much. And we have infixes for expressive purposes in polysyllables: like mathe-freakin-matics, or responsi-freakin-bitity.