What is the difference in use of in- and un- as prefixes?

Answer 1

Un almost always means "not."

unbelievable

unknowingly

In is sometimes just a different spelling of un:

inaccessible

invariant

But sometimes in is an alternate spelling of the intensive prefix en:

infuriate

insure

The two uses of in mean almost the opposite of each other, which leads to one of the most misunderstood words in the English language: inflammable.

Most people think inflammable means "not flammable" or "cannot be set on fire." It actually means "capable of being set on fire" or "very likely to catch on fire." So when you see the word inflammable, it is almost impossible to know what it was meant to mean. Safety professionals advocate just using the word flammable when there is a danger of fire.