While we make an adjective into a substantive of the same meaning by simply substituting one of the endings -e, -o, -a or -um for the adjectival -i, see above under Sex, there is no direct (immediate way of forming adjectives from words belonging to other parts of speech. We must for that purpose use one of the following derivative suffixes.
-AL (ALI) is the most general of these suffixes; it is well known from a great many Latin or Romanic adjectives that have penetrated into most civilized languages and which are used in Novial as regularly formed adjectives: natural, universal, national, verbal, kordial (from kordie heart), material, formal, traditional etc. Before -al the ending of the substantive disappears (cf. also bestie bestial), except only u: from sexu we form sexual, thus also manual and others. We extend this formation to all words, even if there is no adjective with this ending in national languages: gasal relating to gas, patral paternal, populal relating to people, homal human, i.e. concerning man (different from human humane) - on homonal and homanal see above (p. 93).
-AN from L is found in a great many words in Romanic and other languages; it means inhabiting or belonging to a class or party: Roman (Romani, -e, -o, -a), Italian, Amerikan, urban, akademian, senatan, vilajan, partisan, leftano member of the "left" party; further the convenient Esp-Ido formations samlandane fellow-countryman, samreligionano, sampartisane, samideane; also altrilandano, etc.
Roman beleso may be understood as (1) Rome's beauty: then o is stressed, (2) Roman beauty: then a is stressed; in the latter case, when -an is an adj, it is safer to write romani, cf. below on osi.
-ATRI (from F I) similar in appearance or character, often corresponds to the E ending -ish: blankatri whitish, bluatri, metalatri metallike, parodiatri, etc.
-ARI (I -ario, F -aire, etc.) agreeing with or fit for: reglari regular, from regle rule, populari, revolutionari, elementari, ordinari, imaginari (thus Occ).
-OSI (-OS) from L -osus, I S -oso, F -eux, E -ous, ose, D -ös means possessing or having, especially having in great quantity, full of: porosi, kurajosi courageous, danjerosi, misteriosi, gratiosi, graceful, pretiosi, joyosi, barbosi bearded.
With this suffix it is prudent in writing always to retain the adjectival -i, which in most other cases is optional, because here -os might at first blush look like a plural of a sb in -o. In speaking no mistake would be possible, as the accent is different.
-ISI with adverb -isim indicates a very high degree: grandisi enormous, grandisim enormously, belisi, belisim; varmisi hot. This of course is taken from I -issimo, but is here shortened to admit the adverbial -m. This suffix should be used sparingly, not so frequently as Esp -ega, and adjectives in -is- cannot very well be made into substantives in -o, -a (-isa verbal suffix!), so the suffix should be restricted to attributive and predicative uses.
Some suffixes, which serve to form adjectives from verbs, may be termed supplementary participles, some for the active, and others for the passive. To the first class belong -iv and -as.
-IV (-IVI) means doing naturally, or capable of doing (that can ...); from instrukte we form instruktiv(i), further sugestiv, atraktiv, preventiv, atentiv (from atente attend), aktiv, nutriv nourishing (-um nourishment), helpiv helpful, etc.
-ASI (-AS) from F -ace, I -ace, S -az (E sometimes -acious, cf. the sb in -acity) means having the tendency or inclination to ...: mordasi mordacious, snappish, disputasi, vorasi, kredasi credulous, laborasi industrious, atakasi, etc.
This suffix is used in Occ (-aci). Esp and Ido have -ema, coined with some reminiscence of the F verb aimer. One might feel inclined to have two suffixes, -asi for evil propensities (as often in the national languages), and -emi for praiseworthy inclinations, as in laborem (cf. N eme aim), but I dare not propose that. Keep -i in writing -asi, cf. above on -osi.
Next we come to similar supplementary passive participles. -BLI expresses passive possibility: from lekte lektebli legible, from explika explikabli, from audi, audibli, from solu solubli - thus with retention of the vowel of the verbal system. As some verbs may be used both transitively and intransitively, we may have both chanjivi that may change, and chanjabli that can be changed.
Esp has -ebla. Occ has -bil with special rules for the stress and for the vowel and consonant preceding the suffix: from leer leibil legible, from vider visibil, from creder cresibil (?) - as these examples show, not even these intricate rules enable one in all cases to form quite "natural" adjectives, which should, however, be their justification. Much better to have one simple uniform rule, even if we get some words like vidabli instead of visibli.
-ENDI `that must be' and -INDI `that deserves to be,' two suffixes modelled on the L gerund: libre lektendi por examine non es omnitem libre lektindi; amindi lovable, admirindi admirable; me have nulum skriptendi I have nothing to write; vidindum something worth seeing. Before these endings the -u and -i of verbs must be kept: soluendi, soluindi, expediendi, expediindi, but -a and -e disappear.