Perpendicular Actions: Future Continuous Tense expresses an ongoing, continuous action in the future at a defined moment, and this moment is another non-continuous action in the future. Another way to explain:
While an action is ongoing, another action will take place. Both the actions will happen in the future. This second action will not be going in parallel with the first action, because the second action is not a lasting, continuous action, just like the first one. This second action serves a time-indicator. It shows a point in time when the main continuous action will be taking place.
We’ll be studying tenses (at 2 pm / or) when the teacher comes. (it doesn’t matter what indicates a point in time: whether it is a clock or another action)
Why did we use the present simple tense for the second future action?
Interestingly, both the actions happen in the future. However, the one that goes after when, will be a time-indicator for the main action, and it will be used in one of the present tenses. This is because after certain words, we don’t use future line of tenses. One of these words is when.
We’ll be studying tenses at 2 pm / when they come. (it doesn’t matter what indicates a point in time: whether it is a clock or another action)
So, the time indicator action with preceding “when,” will be in Present Simple (not Future Simple).
Remember, a period / point in time can be literally time on a clock or another action. In this scenario, it is another non-continuous action.
Perpendicular Actions: Future Continuous with when / and
Note, we do not use future tenses after:
if / when / after/ before / as soon as / unless / until and their synonyms or synonymous expressions.
Instead, we use present simple tense for non-continuous actions after these words.
That is why:
We’ll be studying tenses when they come.
When they come, we’ll be studying tenses. (pay attention to punctuation)
Both the actions are perpendicular, just because they are not happening continuously and simultaneously in parallel with each other. That’s why they don’t have equal rights, unlike parallel actions. The main action is in continuous line of tenses and the other one is in simple line of tenses.
Instead of “when,” conjunction “and” may be used. In this situation, both the actions use a future tense. This happens because “when” no longer affects the sentence. Compare:
They will be studying tenses when the teacher comes.
They will be studying tenses, and the teacher will come.
In the second example, both the clauses (sentences) use the future line of tenses: one uses Continuous and the other – Simple.
Let’s change this sentence and see how “while” affects it.
While (when) they are studying tenses, the teacher will come.
They will be studying tenses when the teacher comes.
Both the sentences mean the same, however, note how when / while affects the use of a future tense.
This is one of the most difficult topics in studying English tense system. It requires a lot of practice until you do understand it.
Also on Future Continuous: