Some cases:
"When you get the email, forward it to me"
"When the bus comes, hail it to stop"
Does this kind of phrase show certainty that the event the request is based off will happen, or could it be interpreted as just an order to do something in a particular scenario?
takes the occurrence of event A for granted, which is why the construction precludes modal verb will with an epistemic meaning, though this is sometimes seen in conditionals.
It also doesn't allow a counterfactual preterite form, like in a conditional
So I'd say the speaker, in selecting when is making clear that the event in question is certain to happen, at least beyond the need to comment on probability.
(The Cambridge Grammar of the English Language p191)
Non-occurrence of futurity will in temporal clauses
When adjuncts and the complements of temporal prepositions such as after, before, as soon as, etc., take the simple present, not the will construction, when the reference is to future time:
[34]
i I’ll buy one when the price comes / *will come down.
ii We’ll go for a walk as soon as it stops / *will stop raining.
Here I do not assert that the price will come down or that it will stop raining, but simply take the occurrence of these events for granted. The modal qualification which will conveys (in varying degrees) would therefore be out of place: the grammar requires the unmodalised forms.
Futurity will in conditional protases
With future time conditionals the protasis (the subordinate clause) may or may not contain will:
[35]
i If [the price comes down in a few months], I’ll buy one [sc. then].
ii If [the price will come down in a few months], I’m not going to buy one now.
Version [i] illustrates the usual construction: [ii] is quite rare. In [i] I again do not assert that the price will come down (but merely entertain it as a possibility) and therefore the modal qualification associated with will is here too out of place. In [ii], however, will does appear in the protasis because the modal meaning it expresses is part of the proposition that is conditionally entertained. In [i] the condition is a matter of the future occurrence of an event, whereas in [ii] it is a matter of the present predictability of an event (it might be glossed as “If it is the case that the price will come down in a few months”, a type of gloss quite inappropriate for [i]). Note that the behaviour of will here matches that of the other epistemic modals – compare, for example, If the price may come down in a few months, I’m not going to buy one now.