English spelling can be strange. Even if it is logical, there can be differences for no clear reason. I mean, if the plural of ‘mouse’ [see above picture] is ‘mice’, why isn’t the plural of ‘house’, ‘hice’? The plural is actually ‘houses’, but why? Answer: that’s just the way it is.
Speaking of mice (and their bigger relatives, rats) reminds me of another spelling difference which has always confused me. Consider the adjectives in the following two sentences.
He is a desperate criminal.
It’s a separate issue.
Do you notice the difference in the spelling? There a ‘e’ in the middle of the first adjective, and an ‘a’ in the second, but why? Answer: that’s just the way it is.
How do you remember the difference? Well, I always think, …. ‘sepArate the rAt.’ This is called a ‘mnemonic’ = a pattern of letters, words, or ideas to help people remember things.
But do you understand it? When you want to spell the word ‘separate’ (either as an adjective [a separate issue], or a verb [to separate things]), think ‘separate the rat’, and notice that ‘rat’ has an ‘a’ in the middle, and thus so does the word ‘separate’.
I hope this mnemonic helps.