Yeah, that will be difficult to explain! I will just add these comments to the three sentences that *might* help you out. 1) I heard the bomb go off. 2) I heard the bomb went off. 3) I hear the bomb went off. I think you're right about 2/3. In both cases, it came from a source. And they both use the past tense in the second part. Whereas, 1 uses the present tense in the second part and is your direct personal experience. Try adding the word "that". Which forces it to be something you got from a different source. I heard that the bomb go off. <-- doesn't make sense I heard that the bomb went off. <- makes sense I hear that the bomb went off. <- makes sense And the difference in tense in the first part in 2/3 changes the meaning slightly. I hear... --> It is my understanding... I heard... --> Someone told me directly or I read it, etc. And finally, the following doesn't really work in conversation, right? 4) I hear the bomb go off. (this is like you're telling someone your own story) And then... I hope that helps with some kind of explanation. :)

Thank you both!! Very helpful......

You are correct. The second example is called "reported speech." Enter that search term, and Mr Google will tell you all you want to know.