Three examples
1) hacer (do/make), hecho (fact), factible (feasible)
2) reprender (reprehend), reprensible (reprehensible)
3) pensar (think), pensable[concebible] (thinkable), impensable[inconcebible] (unthinkable)
1) few -but important- words are very irregular, as they are:
- in Spanish: words come directly from ancient Italian (wrongly -in my opinion- called "vulgar Latin") so they are what is called palabras patrimoniales, and later the adjectives come by an independent way straight from 1300-1600 Latin.
- in English: words mostly from Germanic origin and a few palabras patrimoniales from old French, and later their adjectives come by an independent way straight from 1300-1600 Latin. Also, similar words with different origins (doable, feasible)
2) a lot of words are very regular, as both the verb and the adjective were taken from Latin in "modern" times.
3) words from different origin but using a suffix: fact
ible (hacer), feas
ible (made), mach
bar (mach)
So, not surprisingly, English and Spanish are not Esperanto so there isn't a way to regularly derive such adjectives, and we even have -able and -ible as suffixes in the regular cases. But it's not hard either.