Answer :

The root word for the word illegal is legal. Notice the prefix il- in illegal. Prefixes are words, letters or some numbers that placed before the word itself. In this case, il- is the prefix of illegal, which means not acquired or done in a moral or official manner contrary to legal, which is the opposed definition of illegal. Examples of prefixes include, re- in remake, re-establish, retake. Notice the root words, make, establish and take in the given words.  



the root word of illegal is legal since in/il means no that mean it's not legal