Indispensable means absolutely necessary; indisputable means factual, without a doubt, and not arguable.
e.g. Air is indispensable to life.
e.g. It is indisputable that the verdict of the judge is final.
Prepossessing / Preposterous
Prepossessing means attractive or impressive; preposterous means absurd or contrary to reason.
e.g. She had put on a prepossessing dress to impress the audience.
e.g. You look preposterous in that ridiculous outfit!
Irritable / Irritant
Irritable means easily made angry; irritant means causing anger or discomfort.
e.g. He has a short temper and is easily irritable.
e.g. Nobody likes him because of his irritant behavior.
Preparation / Preparedness
Preparation means getting ready; preparedness is a state of being prepared.
e.g. The country's preparations for war are complete.
e.g. The country is in preparedness for war.
Inflammable / Inflammatory
Inflammable means easy to catch fire; inflammatory means causing unrest or bad feelings.
e.g. Be careful! This kind of material is inflammable.
e.g. The man's speech was not only anti-government but also inflammatory,
PURPOSELY / PURPOSEFULLY
Purposely means deliberately; purposefully means in a determined way.
e.g. That guy purposely left the trash on the sidewalk.
e.g. The student purposefully worked on his project to get a better score for further advancement.
COMMON SENSE / COMMONSENSE
Common sense is always put in two words. Use a hyphened compound work ass an adjective, and not as one single word.
e.g. Use your common sense when you do this.
e.g. This is just a common-sense approach to the problem.
ALLOW / ALLOW OF
Allow means permit; allow of means leave room for.
e.g. The new regulation will not allow you to do this.
e.g. The procedure is so precise that it will not allow of any variation.
Copyright© by Stephen Lau