:up:Finally, I understand.
I had problems with this throughout my career. I could not find a straightforward explanation that covered every possibility, until I discovered Lewis.
One of the key things is "
The restriction may be a real one [...] or a psychological one existing only in the mind of the speaker." Two native speakers may look at an identical situation; one will come up with an utterance containing
some, the other with one containing
any.
Like so many things in English (the choice of the 'correct' way of expressing the future is one of these), we are in the area of
grammar as choice. Unfortunately some teachers and course books often give the impression that one way is 'better' or 'more natural' than all others. It may seem more natural to one speaker, or even to many speakers; that does not necessarily mean that a speaker who looks at the situation in a different way is wrong.
It is also unfortunate that many learners are unhappy about this. They would prefer a firm rule. Actually, once one grasps the concept of
grammar as choice, it is very liberating. That nagging thought one had that another way seemed possible can be true - despite what the teacher may have claimed.