If something cannot be explained, it's... (2x)
Unexplainable / Inexplicable
If something cannot be defended, it's...
Indefensible
If someone cannot be defeated, they are...
Unbeatable
If something is impossible to endure or withsstand, it's...
Unbearable
If something cannot be replaced, it's...
Irreplaceable
If you can't read something, it's...
Illegible
If you can't overcome or be better than someone, they are...
Insuperable / Insurmountable
If someone never gets tired, they are...
Indefatigable
If you can't understand something that you're hearing, it's...
Unintelligible
If you can't hear something, it's...
Inaudible
If you can't correct / reform someone, they are...
Incorrigible
If something cannot be described using words, it's...
Ineffable
If you can't eat something, it is...
Inedible
If something cannot catch on fire, it's...
Incombustible
If something is too precious to be bought with money, it's...
Invaluable
If something cannot be imagined, it's... (2x)
Unimaginable, inconceivable
If someone cannot be managed, they are...
Unmanageable
If something can't be avoided, it's.. (2x)
Unavoidable / Inevitable
If you can't physically touch something, it's...
Intangible