I hear that - I have heard that - I heard that
I heard means “heard” something from a specific source (a friend, newspaper, magazine, internet, etc). It may have been once or multiple times.
I heard the president had got divorced.
I hear means the rumour is… the common knowledge is…
I hear, there is measles going round. It happens «from time to time», «here and there”.
I have heard means that something has once happened in the past and is finished, it is not expected to be heard again any time soon.
I have heard that you are leaving soon.