Sometimes we use the Present Simple tense for future meaning, particularly with schedules and appointments. For example: Cassie has a doctor’s appointment at 3pm tomorrow, or The plane arrives at 11:20. Other times, we use the Present Progressive tense for future meaning, especially when expressing set plans, typically ones involving other people. For example: I’m seeing the latest action film with my friends tonight!
For native English speakers, this seems normal. But when learning this (or teaching it for the first time, for that matter), it seems a bit odd. Why would we do this? It goes against our usual standards for expressing when things happen, right?
Not exactly. The way that English is constructed, we don’t actually change the form of the main verb from its present form in order to express the future; instead, we add a modal to the verb string, which qualifies it in various ways, typically by expressing that the verb has possibility.
The present form essentially communicates that the verb isn’t in the past. If there is no modal on it, we take that to mean it is true and real. That level of uncertainty almost always implies the present (that certainty also works for the past, but that’s why we have a different form to differentiate the two). The future is generally uncertain, which is why we use modals. Even when a verb becoming true/real is a very, very strong possibility (in which case we’d use will or be going to), there’s a slight chance that it won’t come to pass for whatever reason.
There are exceptions, however, as with appointments, schedules, and set plans. When we express these verbs, by default they have certainty; we don’t generally consider that they might not be true (or if we do, we’ll need to add more words, phrases, or clauses to say so). To say something with the present simple or present progressive tense is to consider its truth/reality as sure as a verb happening in this very moment.
So next time you’re comparing and contrasting present and future forms, instead of considering when they happen, consider whether we’re utterly certain that they happen at all.