Main principle: Future Continuous Tense shows that an action will be happening over a period of time in the future.
We’ll be watching movies tomorrow.
In non-continuous tenses like Future Simple, the action is not ongoing.
They will accept our offer.
Some actions might sound ongoing:
They will go to the store tomorrow.
However, in the above sentence, the stress is on the end of the action, on it’s final result, and not on the length of the action. To better understand it, we can use the inversion trick.
They will go to the store tomorrow = They will come to the store tomorrow.
Take a look at the example with movie watching. Some would say that this sentence
We will watch movies tomorrow.
expresses an ongoing, continuous action. However, it’s not true. We can invert the action to it’s conclusion, to the end of the movie activity . As a result, a finished activity wouldn’t seem ongoing or continuous.
Another way to explain this situation is that we have fact of action vs duration of action. Compare:
We will watch movies tomorrow. (fact of action)
We will be watching movies tomorrow. (duration of action)
But how do we know that there is a length of an action, that an action is continuous, ongoing? Well, for this we normally have explicit or implicit indication of duration:
Tomorrow (meaning a significant or some part of it), all day, all morning, for a few hours, from 5 to 6, during his visit, etc. etc.
Anything that can show duration. If we don’t have indication of duration, we infer this duration from the context. We call it contextual point or period in time.
Also on Future Continuous:
Do you still have questions?