According to the Oxford Grammar Course (Intermediate) book by Swan & Walter, we normally use can if we are deciding now what to do in the future. In other cases, we use will be able to.
I need some help with the questions number 8, 10 and 11 in the first exercise of the page:
8 I'm free at the weekend, so the kids __[can]__ come round.
10 We're busy this week, but we __[can]__ repair it by next Thursday.
11 I __[can]__ pay on Saturday – I promise.
I put can, as indicated here.
My reasons for all three:
8) I think can is correct because the speaker is deciding to allow the kids to come round in the weekend though he is busy.
10) I think can is correct because the speaker is deciding when exactly what to do the next Thursday.
11) I think can is correct because the speaker is deciding (and consequently promising) what to do on Saturday.
But the answer key says these three sentences should have will be able to.
I really hope that there is someone here who has a deep knowledge of grammar and can explain to me why we can't use can in these three questions. Why do we have to use will be able to?