I always thought that is something can be done then I can say e.g. alterable or acceptable or believable.
When I want to say the opposite of them, then I only have to add “un”. like unalterable, unacceptable, unbelievable.
But now I see that there’re words which use “in” instead of “un” like indescribable or inaccessible 
Is there a rule for using “in” or “un”?
Hi
Oh, yes…
As far as I know from my tutor, there is no formal rule – you have just to learn the opposite forms by heart -
except for words that begin with the same letter as a possible prefix finishes:
irresistible, illegal, immortal, etc.
Tamara
P.S indescribable is correct.
Hi Hon 
As Tamara mentioned, there are several prefixes that are used to add the concept of not to a word — in other words, to create a word with the opposite meaning. For example:
un-, im-, il-, ir-, in-
There is no “100%” rule about using these prefixes, but there is a little help with “half-rules” (so to speak). Some of these prefixes are used only with words that begin with a certain letter.
im- This is only used with words (but not all of them) that begin with M or P:
immeasurable, impolite, impossible BUT unmasked, unperturbed
il- This is only used with words (but not all of them) that begin with L:
illegal, illegible, illogical BUT unloaded
ir- This is only used with words (but not all of them) that begin with R
irreplaceable, irrelevant BUT unreasonable
The prefix in- has no “half-rule” whatsoever as far as I know, but it is often used with words beginning with a vowel:
inadmissable, inedible, inexplicable, inoperable
And, of course, there is also the prefix dis-: dissastisfied
There are only “half-rules” that you can use as a very general guideline for these prefixes. Unfortunately, there are no 100% rules.
Amy