Today we have three very different words, spelled differently and with different meanings. I think most people, if they stop and think, could define each one individually. But where I see people constantly coming unstuck, even venerable institutions that should know better, like authors with several books published, newspapers and the like, is with the phrases in which these words are commonly used.
Some definitions first.
Rain is precipitation of non-frozen nature – water that falls out of clouds.
A rein is a leather strap attached to a bridle, with which you guide a horse’s direction, either from on its back (usually reins) or walking by its side (a lead rein) or standing in the middle of a paddock with the horse going in a circle around you (a lunge rein).
A reign is the period of time during which a monarch (or, by extension, any high-up leader) rules over their people (or, in the extended version, their social group, organisation, etc.)
So far so good. So, those tricky phrases. In fact, you can work out quite easily which one should be used where, by considering the literal meaning.
“Don’t rain on my parade” – don’t ruin my fun (I’m having a parade up the main street of town, if it rains it’ll get all soggy and ruined and everyone will go home – no one’s ruling anyone (reign) and there are no leather straps involved (rein)).
“We’ll have to rein him in” – he’s out of control and we need to limit what he’s able to do and bring him back under control (we need to have a little pull on the reins and stop the horse running too fast – we’re going to rule him but not in the sense that we’re a monarch and he’s one of our subjects (reign), and he’s not going to get wet (rain))
“A reign of terror” – something or someone is making things rather uncomfortable for everyone else (they are reigning over something that they are able to rule and control – no one is getting wet (rain) and we’re not frightening any horses (rein))
So that should be all clear now, right?