1. Discreet - indiscreet

Note: Discreet means to be careful and prudent in one’s words and actions so as not to reveal something that ought to be kept a secret or to avoid embarrassment. Adding the prefix ‘in’ to the word ‘discreet’ we get the word ‘indiscreet’ which means being irresponsible or unwise in revealing secrets too earlier than they should be told.

2. Legal – illegal

Note: The word legal means pertaining to the law. Adding the prefix ‘-il’ to the word, we get ‘illegal’ which means contrary to or forbidden by the law.

3. Continue - discontinue

Note: To continue means to proceed with an activity, or resume it after a break. Adding a prefix ‘dis’ to it, we get the word discontinue which means to stop doing something that one was doing, maybe on a regular or continuous basis.

4. Regular - irregular

Note: The word regular generally means usual, customary or even uniformly arranged. Its antonym is irregular, which can be used to refer to either asymmetrical shapes or something that is not uniform.

5. Equal – unequal

Note: the word equal can refer to a similarity between two people or objects. Its opposite can be derived by adding the prefix ‘un’ before it, to mean some kind of disparity or dissimilarity.

6. Usual – unusual

Note: the word usual denotes anything that is customary or ordinary, whereas its antonym, which is derived by adding ‘un’ prefix to it, refers to something strange or odd, something that is not customary.

7. Advantage – disadvantage

Note: the word ‘unadvantaged’ is often used in place of disadvantage, which is the right option. The word advantage denotes some form of merit or benefit, while the opposite is disadvantage.

8. Mobile – immobile

The word mobile doesn’t refer to the phone, but refers to any person or object that is moving. The opposite is immobile, which refers to something that is stationary.

9. Correct – incorrect

10. Pleasant – unpleasant

11. Logical – illogical

12. Relevant – irrelevant