@WBYeats,
Quote:For
I'm so glad that you could come.
Can COULD be replaced by CAN? I think not, because it seems to refer to a past thing, but I'm not sure....
I'm going to guess that this example, which I've put in blue is being spoken at this moment, as in,
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[Mary walks into John's house for a party that she wasn't sure she would be able to attend]
John: Heeeelllllooooo Mary, what a nice surprise! I'm so glad that you could come.
Mary: Thanks, John. I'm glad to be here. I didn't/don't have to work late after all.
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I really can't think of anyway that 'can' could be used in this circumstance. However, that doesn't mean that COULD is the past tense of CAN. That simply means that this is one of the meanings that COULD fulfills.
A possible paraphrase for,
I'm so glad that you could come is,
I'm so glad that the conditions exist at this time for you to be able to come to my party.
As you know, I gave a description, there are situations for the present where COULD, MAY & MIGHT all work, but CAN does not. That's two purported past tense modals - COULD & MIGHT - working in a present situation.
Again, it's a semantic connection, not a syntactic one. And this "close" similarity is only really apparent for CAN & COULD because they share the same wide meaning - IT'S POSSIBLE.
Let's look at that same situation, with a twist.
[Mary walks into John's house for a party that she wasn't sure she would be able to attend]
John: Heeeelllllooooo Mary, what a nice surprise! I'm so glad that you could come.
Mary: Thanks, John. I'm glad to be here. I didn't have to work late after all.
1) John [on the phone to JTT]: Jtt, guess what! Mary was able to come to my party after all.
[COULD is not possible here]
2) John [walks into the kitchen and says to some other friends]: Tom, Jill, guess what! *Mary could come to my party after all.*
[*------* denotes ungrammatical for the situation]
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[Mary walks into John's house for a party that she wasn't sure she would be able to attend]
John: Heeeelllllooooo Mary, what a nice surprise! I'm so glad that you could come.
Mary: Thanks, John. I'm glad to be here. I didn't have to work late after all.
[Next day, John phones me [JTT] to talk]
John: JTT, guess what! Mary was able to come to my party after all.
JTT: John, Mary
can't have been there at your party last night. She was with me all day and all night yesterday. That was an imposter.
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Notice the use of CAN'T in this clearly past tense/past time situation. Yes, COULDN'T is also possible but it doesn't have the same strong denial as CAN'T. This is because CAN is a more reality based, a more certain IT'S POSSIBLE than COULD is.
[COULD is also not possible in the blue sentence above]
John: JTT, guess what! *Mary could come to my party after all*.
[*------* denotes ungrammatical for the situation]