When you are introducing two sides of an argument, it makes sense to use the term ‘side’ – on one side this, on the other side that. But unfortunately, it doesn’t work in English.
We use the word ‘hand’. Why? Well, think of a typical English man – your teacher perhaps… Imagine him weighing up two options – he’s probably got his hands out, right? Left hand for one option, right hand for the other. Us English love to use gestures.
So that’s what we say – on one hand, this, on the other hand, that.
Also notice the use of ‘the’ in ‘the other’ – this is important. If you omit the ‘the’…well, it sounds like a foreigner trying to speak English. ‘On one hand…on other hand‘ – nope, definitely not a native.