Incorrect spelling, explanation: while in many cases the process of adding suffixes is very straightforward, in the cases when a word ends in e and the suffix starts with a, instead of compounding e and a together (which happens really rarely), the easier route was to get rid of the e, especially if it’s silent. This is why believeable, even though it may look believable, is not a correct word.

Correct spelling, explanation: believable is formed by suffixing the verb believe with able. It might seem like the end word could be believeable, but when you look at English vocabulary, ea is not found that often, especially when it comes to suffixes. Because the e at the end of believe is silent anyway, it’s exchanged from the a from able. It’s also much more comfortable to look at.

Definition of believable:
Adjective
1. Something that can be regarded as possible, making sense, or trustworthy.
It seems believable that this accident happened simply because of the weather.
He felt that her story was not quite believable.