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He could not turn up is used if discussing something that happened in the past. For example, "His car broke down last night so he could not turn up". He might not turn up means similar but it's spoken in the present. So if two people are discussing the situation now, one would say, for example, "it's getting late, he might not turn up".

But in your case, might is not really the right word either. I think you would say "let's not wait any longer, it doesn't look like he's going to turn up". If you say he might not turn up, it just sounds wrong. It's as though your saying "He might turn up or he might not, but we won't wait any longer anyway"

He could not turn up is used if discussing something that happened in the past. For example, "His car broke down last night so he could not turn up". He might not turn up means similar but it's spoken in the present. So if two people are discussing the situation now, one would say, for example, "it's getting late, he might not turn up".

He could not turn up is used if discussing something that happened in the past. For example, "His car broke down last night so he could not turn up". He might not turn up means similar but it's spoken in the present. So if two people are discussing the situation now, one would say, for example, "it's getting late, he might not turn up".

But in your case, might is not really the right word either. I think you would say "let's not wait any longer, it doesn't look like he's going to turn up". If you say he might not turn up, it just sounds wrong. It's as though your saying "He might turn up or he might not, but we won't wait any longer anyway"

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He could not turn up is used if discussing something that happened in the past. For example, "His car broke down last night so he could not turn up". He might not turn up means similar but it's spoken in the present. So if two people are discussing the situation now, one would say, for example, "it's getting late, he might not turn up".