Words starting with "m":
* immature: not mature
* immoral: not moral
* immobile: not mobile
* immortal: not mortal
* immaterial: not material
* immeasurable: not measurable
* immigrate: to move into a country
* imminent: about to happen
* impassioned: not passionate
* impatient: not patient
Words starting with "p":
* impossible: not possible
* impolite: not polite
* imperfect: not perfect
* impotent: not potent
* impractical: not practical
* impressive: not impressive
* improbable: not probable
* impure: not pure
* impoverished: made poor
* imprison: to put in prison
Other examples:
* imbalance: lack of balance
* imbecile: a person with a low IQ
* imbue: to fill with something
Note: Some words can also use the prefixes "in-", "un-", or "dis-" to create a negative meaning, depending on the word's origin and historical usage.