Common Words:
* Illegal: Not legal
* Illiterate: Unable to read or write
* Illogical: Not logical
* Illusion: A false perception
* Illness: A disease or sickness
* Illustrious: Very famous or distinguished
* Ill-tempered: Bad-tempered
* Ill-fated: Doomed to misfortune
* Illegible: Difficult or impossible to read
* Illicit: Not permitted or legal
* Ill-timed: Happening at an inconvenient time
Less Common Words:
* Illapse: A sudden collapse or failure
* Illapse: To fall or slip into a state
* Illicitly: In an unlawful or illicit manner
* Illume: To illuminate
* Illiberal: Narrow-minded or intolerant
* Illude: To deceive or mislead
* Illustrative: Serving to illustrate or exemplify
* Illumine: To illuminate or enlighten
* Illusional: Relating to or resembling an illusion
* Illusory: Based on or producing illusions
* Illy: Badly or poorly
This is not an exhaustive list, but it gives you a good starting point for understanding words with the prefix "il."