Prefixes: des-, re-, in-, pre-

In- reshapes itself to fit the next letter

In- is the main negating prefix, and its trick is that it changes shape to match what follows. It stays in- before most letters, becomes im- before p or b, and assimilates fully to i- before l or r. Only the spelling moves; the meaning never does. English does exactly the same thing, so your ear already knows the tune.