Explain why some words use the prefix 'im' instead of 'in'?

Answer 1

The prefixes il-, ir-, im- and in- all mean "the opposite of". Which ones is used is based on the base word and that word's initial letter.

You use im- instead of in- if the base word begins by "p" or "b". For example:

Imberbe

Imposible

but intacto.

Answer 2

The prefix 'im' is used instead of 'in' before certain consonant letters to maintain pronunciation ease. When followed by 'm', 'b', or 'p', 'in' changes to 'im' for smoother pronunciation, such as in words like "impossible" or "important".