Teacher: Yes, that’s correct: We say “In the future, the planet will be warmer.” So when we’re talking about the future, we use will to…
Student: But teacher you can’t!
Teacher: Excuse me?
Student: You can’t say will because you’re not sure! What if things change? You have to use might, because maybe it won’t be warmer in the future! You can’t be certain!!
Teacher: Ok, tell me: what day will it be tomorrow?
Teacher: Ah, but how can you be sure it’ll be Saturday? What if the world ends this evening?
They usually get it at that stage. I imagine that this is quite a common problem for many teachers when trying to teach how to use will. It’s funny, because there’s a logic to students asking this. How can we be sure of what’s going to happen in the future?
Of course we can’t, not really. If we really thought about it, we’d never be sure of anything yet to happen, and we’d be full of doubt about. To make life easier and stop our brains from thinking too much, we just assume something will happen if we have enough evidence, past examples, or logic to be reasonably sure that it will happen.
We do the same thing with language: instead of just using might or maybe all the time, we use will, or going to, or the present continuous. It’s just simpler. And yet even when non-English speakers do something similar in their own languages, they sometimes still get caught up on using might all the time. It’s a classic case of getting bogged down in the strict rules of a language instead of considering how the language (and their own language!) is actually used in real life. People learn at an early stage that we use might when we’re not sure, and will when we’re sure (about the future), so they follow that rule unquestioningly. Even when they use will all the time to refer to the future in their natural speech (in fact, most students overuse will), in the classroom they revert to focussing on the grammar rules. The language stops becoming something to actually use in real-world situations, and instead becomes something more abstract which exists only to provide correct answers in a lesson.
That’s a problem with learning a language that’s very hard to overcome: to learn it one has to step back and analyse it. And once one does that, it can be easy to forget that a language is a real, living, evolving thing, that is the most common way for us to transmit our thoughts to others, as opposed to an abstract subject of study.