Indescribable or undescribable

[Attila]

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"?

[Tamara]

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.
_________________
It’s impossible to learn swimming without entering the water…

[Yankee]

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