What's the difference between "unqualified" and "disqualified"?

Because unqualified and disqualified have the same root, and the prefixes un- and dis- seem to have very similar meanings, one might think that unqualified and disqualified mean something similar. However, their meanings are quite different, and when we examine the parts of these two words we can see why.

The prefixes un- and dis-

First it is important to understand that un- and dis- mean "not" when they are attached to adjectives (for example unreasonable means "not reasonable" and disagreeable means "not agreeable"), but they mean "do the opposite of the verb" when they are attached to verbs. For example, untie means do the opposite of tie, and disappear means do the opposite of appear. 

Unqualified

Unqualified is made up of the adjective qualified, which means "having the necessary skill or knowledge to do a task" with the prefix un-. Because qualified is an adjective, un- here means not, and the whole word means "not having the skills or knowledge needed to do a task." 

Below are some example sentences that illustrate the meaning and use of unqualified: not having the skills or knowledge needed to do a task.

• He is clearly unqualified for the job. 
• We received job applications from many unqualified candidates. 
• It's a judgment that you are unqualified to make.

Disqualified

Disqualified comes from the verb qualify. Because qualify is a verb, dis- here means do the opposite of qualify. Qualify means "to give someone the right to do or be a part of something" and therefore disqualified means "taking away from someone the right to do or be a part of something."

Below are some example sentences that illustrate the meaning and use of disqualified: taking away from someone the right to do or be a part of something.

• The winner was later disqualified for cheating. 
• His poor eyesight disqualified him from becoming a pilot.  
• After Tyson bit the other fighter's ear, the judges disqualified him from the boxing match.

The most important thing to remember is that the exact meaning of the prefixes un- and dis- depends on what kind of word they are attached to: adjective, verb, or noun.

I hope this helps.