The prefix in~ means not or the opposite off. That's why it is really useful for people learning English to build up their vocabulary. It is like the Japanese prefix FU, 不.
Depending on the sound that follows in~ might become il~, ir~ or im~. Check out this website for a more detailed explanation: http://www.spelling.hemscott.net/prefix3.html
Now let's look at some examples~!
expensive, 高い <------> inexpensive, 安い
You could also say cheap as an opposite of expensive but inexpensive sounds more polite and less direct. Also cheap can be kind of negative. I guess 高くない is closer to inexpensive in terms of connotations.
admissible, meaning CAN be admitted <------> inadmissible, meaning CAN'T be admitted
The evidence was properly collected and so is admissible.
I'm sorry you're too late your book is inadmissible for the contest.
possible ,可能 (かのう) <-------> impossible, 不可能 (ふかのう)
In Japanese, the prefix FU~ is acting in the exact same was as IM~!
It's possible that someday people will walk on Mars.
It's impossible to eat 5000 donuts at one time~!
reparable , meaning CAN be fixed <------> irreparable, meaning CAN'T be fixed
Is my car reparable or do I need a new one?
I'm sorry but the damage to your heart is irreparable...
legal, 適法行為 (てきほうこうい) <------> illegal, 不法行為 (ふほうこうい)
Smoking marijuana in Amsterdam is legal.
It's illegal to smoke cigarettes in a bar in most of Canada.
I hope that helped~! Expect more lessons next month,
Phil