[Conscientious]

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Good Q

Ok for these prefixes (il-, ir-, im- ) u don't have a choice , u have to adhere to the rule which says

Words which start with 'm' or 'p' take the prefix 'im-' (impatient)
Words which start with 'r' take 'ir-' (irresponsible)
Words which start with 'l' take 'il-' (illegitimate)

However , the prefixes ( in- , un- ) don't have a certain rule but see what some teacher told me

So, the more work you do in understanding where words came from, the more accurately you can guess whether to use 'in-' (which becomes 'im-' 'ir-' or 'il-' as explained in the first reply) or 'un-'. Your first language may help you guess (more so if it's French or Spanish than if it's Chinese!)a


Got it

good luck