What is the morphology of the word sustainability? I immediately see the -ability suffix on the end of the word as well as a potential su- or sus- prefix on the beginning. Both su- and sus- are assimilated forms of the sub- prefix, leaving <Stain> or <Tain> as the base of the word.
To determine the morphemes of sustainability, I must investigate the etymology. According to all the dictionary sources that I checked, the word comes from Modern English sustain + -ability, which confirms the -ability suffix denoting “capableness, possibleness.”
<Sustain + ability → sustainability>
The remaining <sustain> forms the verb sustain from Middle English susteinen, sustenen, which comes from Old French sustenir, which comes from Latin sustinēre. The Latin verb sustinēre consists of sub– + tenēre. As I initially suspected, the <sus> is the sus- prefix, an assimilated form sub- before <s> denoting “under, beneath, below, behind, resulting from further division, at the foot of, close to, up to, towards, within, during, subject to, in the power of, a little, somewhat.”
The base of sustainability is therefore the bound base <Tain>, which ultimately comes from Latin -tinēre, the combining form of tenēre meaning “to hold.” The combining form -tinēre passed through Old French tenir, which then entered Middle English as -teinen and -tenen. The spelling change to <Tain> with the <ai> in Modern English reflects the phonological change from [ɪ] in Latin -tinēre to [eɪ] in Old French tenir. The <ai> in the Modern English sustain spells the [eɪ].
Other morphological relatives with the <Tain> base include abstain, container, detainee, entertainment, maintain, pertain, retain, and unobtainable, among others.
<abs + Tain → abstain>
<con + Tain + er → container>
<de + Tain + ee → detainee>
<Ent(e)r + Tain + ment → entertainment>
<Main + Tain → maintain>
<Per + Tain → pertain>
<re + Tain → retain>
<un + ob + Tain + able → unobtainable>
Etymological relatives of the <Tain> base include <Tene> as in tenable and maintenance from Latin tenēre, <Tent> as in content and abstention from Latin tenus from tenēre, and <Tine> as in abstinent and continue from Latin -tinēre from tenēre.
The morphemes in sustainability are thus the prefix sus-, the bound base <Tain>, and the suffix -ability.
