You are welcome to answer questions posted in the Ask a Teacher forum as long as your suggestions, help, and advice reflect a good understanding of the English language. If you are not a teacher, you will need to state that clearly at the top of your post. Please note, all posts are moderated by our in-house language experts, so make sure your suggestions, help, and advice house the kind of information an international language teacher would offer. If not, and your posts do not contribute to the topic in a positive way, they will be subject to deletion.
Generally speaking, there is no real rule that you can memorize. That is, there is no pattern that tells you whether an antonym can be formed with im-, un-, in-, etc.
Sometimes there seems to be a pattern...
impossible
improbable
impatient
but then comes
paid >> unpaid
and spoils it
Overall, you just have to memorize them "as they are".
Antonyms Problems 
Linear Mode
