It's not that I can't memorize them, but I have always been wondering how it has become the way it is - what's the purpose of differenting them?
Why not just always use the singular form?
I am a native Chinese, and in Chinese verbs don't differentiate singular/plural forms and even tenses.
I understand tenses can be useful, as they indicate the time of the action. But since the subjects can already indicate if the number is one or more, why not just use one certain form under all circumstances?