When I am using the future tense, why do "while" and "when" clauses have to be in the present tense?
For instance,
"While I am eating you will be speaking on the phone"
and
"I will be waiting when your plane arrives"
Both sentences are grammatically correct.
But, doesn't it make more sense to say
"While I will be eating you will be speaking on the phone"
and
"I will be waiting when your plane will arrive." ?
'While and When' are subordinating time conjunctions like before, after, ere, till etc. Particularly, these two conjunctions, amongst others, also mean 'during the time that' and connect two events happening at the same point of time. When these conjunctions, or for that matter, other time conjunctions refer to future time in a dependent time clause, we do not use 'shall/will' but stick to Present tense/Imperative. Cf.
You will push the button and the door will open. X. When you push the button, the door will open.
Why?
Broadly speaking, English Tenses rest on Time(when) & Aspect(how) where sequence has a vital role to play. In present and past tense, when & while pose no problems, only sequence is the deciding factor.
The restriction applies to future tense (use of will). Simple present is very common in future references when time of the subordinate clause is anterior to or simultaneous with that of the main clause. Present can foresee near future of firm decision, scheduled action or fixed time table.
So to say, a subordinate clause is subservient to the verb (predicator) of the main clause. Actually, future time is determined by the main verb of the main clause. If the main clause already has future reference, it is not usually necessary to specify this again. There is moreover the question of sequence/ correlation.
Wouldn't it be judicious to let the volcano erupt first.
Admittedly there may be situations where it becomes necessary to imbibe future time reference in the subordinate clause also. ln that case we may make use of the peripheries "going to..." instead of an explicit future. But mostly we would stick to the thumb rule of using present tense.
The example sentences with the subordinate clauses in the present are grammatical — sequentially and semantically.
I think it's helpful to consider the future tense first.
I will be waiting when your plane arrives.
Imagine that you have traveled in time into the future. You are now in the future, and you are waiting at the airport. (Hopefully you have a smartphone so you can play games.) What happens now? The plane arrives now. When your plan arrives is a temporal prepositional phrase. It's unnecessary to use the future tense in this part of the sentence. Because you've traveled to this future moment in time, it's obvious that whatever happens now will be present tense.
Yes, it does make some sense to put both phrases in the future tense, but it's unnecessary and a little cumbersome. Once you get used to the idea of the temporal phrase being present tense, it's easier to separate the ongoing event (I will be waiting) from the event that happens in the moment (your plane arrives). It suggests that two events usually don't happen in the exact same moment, and that one event usually has a longer duration than another.
English technically doesn't have a future-tense conjugation although almost everyone, including me, calls it the future tense when "shall" and "will" are involved; however, there is no future conjugation of verbs per se. For instance:
Infinitive: to do
Simple Present Indicative: he does
Simple Past Indicative: he did
Simple Future Indicative: he will do
Emphatic Future Indicative: he shall do
The future-tense inflections haven't become lost since Old English; they've never existed. Because there has never been a verbal conjugation for the future in English, other verbs have had to be borrowed and modals have had to be formed to express this, but how? Well, in Old English, there were two verbs whose infinitives were
sculan ("have to" or "to owe")------shall
and
willan ("to want" or "to wish").-------will
Can you guess what words these are in Modern English? You guessed it: shall and will. These verbs were the closest things the Anglo-Saxons had to expressing the future. Obviously,
"I have to do this." = "I shall do this."
"He wants to do this." = "He will do this."
That's not really much of a future tense, is it? But that's what the Anglo-Saxons were stuck with because they had never thought to devise a future tense for their language. It wasn't their fault. The Angles, Saxons, and Jutes were Germanic tribes that invaded England at the beginning of the fifth century A.D. and, of course, since they were Germanic, that meant that they brought Germanic dialects with them replete with Germanic grammatical rules. Germanic languages do not have a future tense; Modern-day German has to do what English does to talk about the future. Talk about living in the past. Why don't Germanic languages have future tenses? I don't know; ask the Germans. Maybe the past was more interesting to them than the future because the future was uncertain; the past, though, could never be taken away. That's just my educated guess, so don't hold me to it.
So now to get back to your question: Why do "while" and "when" have to be in the present tense? Well, because there was no future tense back in Old English so that's what they had to do. "While" may have had some use of the present subjunctive as well, much like other subordinating conjunctions such as "until", "before", and "if". "When" had some use of the present subjunctive, but not consistently, and more often the subjunctive was used when the subordinating conjunction "whenever" was used rather than its namesake "when". Some examples of this, which are somewhat archaic or very formal, are:
"I shall be there waiting for James, whenever he [shall] arrive."
"Whenever you [shall] be done, Jack will pick you up."
"I shall go with you when you leave, whenever that [may] be."
NOTE: [ ] around a word means the modal used is optional and one could just use the present subjunctive form of the verb if he so wanted.
Based upon my explanation above, I think you'll find that this is the most plausible answer to the question you have posed.
