These suffixes build formal, academic words, so learn the ending AND what the whole word means. -ology names a field of study: psychology is the study of the mind and behaviour. -ism names a belief, a practice, or a condition: capitalism is an economic system in which private owners, not the state, control trade and industry for profit, while racism names a prejudice and criticism names a practice. -ist names the person tied to that field or belief: a psychologist is an expert who studies the mind and behaviour. Match the ending to the meaning you want.
Naming fields, beliefs, and people
psychology /saɪˈkɑlədʒi/ — She switched to psychology because she wanted to understand how memory works.
capitalism /ˈkæpɪtəlɪzəm/ — The essay compares capitalism with earlier economic systems.
psychologist /saɪˈkɑlədʒɪst/ — A psychologist reviewed the survey before we published the results.
Not ""Psychism is the study of the human mind." (-ism names a belief, not a field of study)" — say ""Psychology is the study of the human mind." (-ology names the field)". -ology = the field of study; -ism = a belief system. Swap them and you name the wrong kind of thing entirely.
Try it: Which word correctly names the academic field that studies society? (sociology / socialism / socialist) Answer: sociology — 'Sociology' uses -ology, the ending for a field of study, so it fits 'the academic field that studies society.' 'Socialism' with -ism names a political belief system, not a field, and 'socialist' with -ist names a person who holds that belief.
Building abstract nouns
To name a quality or state, adjectives take noun suffixes. Learn the whole noun's meaning, not just the ending. -ity turns many adjectives into nouns: flexibility is the ability to change or adapt easily, and complexity is the state of having many connected, hard-to-follow parts. -ness does the same for other adjectives (kind to kindness). -ship names a status or role: leadership is the position or ability to guide a group. A rough guide when you are unsure: long, Latin-looking adjectives usually take -ity (complex, flexible, stable), short everyday ones take -ness (kind, dark, happy). When in doubt, -ness is the safer guess.
flexibility /ˌflɛksəˈbɪləti/ — The new schedule gives staff more flexibility to work from home.
complexity /kəmˈplɛksəti/ — The complexity of the tax rules confuses even the accountants.
leadership /ˈlidərʃɪp/ — Strong leadership kept the team calm during the crisis.
stability /stəˈbɪləti/ — The new contracts brought some stability to the department.
Not ""The complexness of the problem surprised us." (wrong suffix for this adjective)" — say ""The complexity of the problem surprised us." (complex takes -ity)". Long, Latin-looking adjectives like complex take -ity, not -ness. 'Complexness' isn't the standard word, when the adjective looks like this one, reach for -ity.
Try it: Which is the correct abstract noun from the adjective 'stable'? (stability / stableness / stableship) Answer: stability — 'Stable' is a long, Latin-looking adjective, so it takes -ity: stability. 'Stableness' uses -ness, which suits short everyday adjectives like kind or dark, not this one. 'Stableship' misuses -ship, which names a status or role, not a quality.
Try it: Which suffix best names a status or skill, as in 'the state of being a member'? (membership / memberity / memberology) Answer: membership — '-ship' names a status or role, so 'membership' correctly means the state of being a member. '-ity' in 'memberity' builds a quality noun and doesn't fit, and '-ology' in 'memberology' would name a field of study, which makes no sense here.