Is/was

Is/was
 

Someone's presentation ended less than several minutes ago. We are in discussion time. Could we use 's?(=is, not was)

-It __ a good presentation.

The trouble here is, in English, even when something happened in the past, if it is recent, sometimes the present tense is still possible. So I am not sure whether is is wrong.

If he continues his presentation periodically, you can say, it's a good presentation. If it is just once, and it's over, then it way a good presentation is the best choice.
Incidentally. the past tense of the contraction it's is 'twas. 'Twas is very old fashioned however. To American ears. it sounds like 19th century British English, even though it's probably used in contemporary British speech.

Good answer. Thank you.

Another time you can use "It is a good presentation" is a situation where the speaker is giving you the presentation to judge the presentation itself, and not the content of the presentation.

For example, if someone is trying to sell me some soap, and they gave a presentation, I would say, "That was a good presentation. You have convinced me to buy soap."

But if they are testing the presentation out, because they are going to be trying to sell someone else soap tomorrow, I could say, "That is a good presentation. I think you can make it shorter, but it's fine as is." Because I'm not talking about the event that happened (the presentation), I'm talking about the presentation itself, which still exists: you could give it again.

That said, in this situation, if you used "was" it would have the same meaning.

Another example. If you show me your dog is well-trained, I would say, "That is a well-trained dog." Because I'm talking about the dog, and the dog is still well-trained as I speak. If I said, "That was a well-trained dog," it would mean the dog is no longer present -- he left.

Excellent answer. Thank you.