Eg I sent it yesterday/ We sent it last week. => There is a rupture between today and yesterday/last week - simple past
Eg I have just sent it/ We have sent it in the morning. => There is no rupture as the day is the same - present perfect.
Pay attention, it is getting common in spoken English to use the simple past while it should be the present perfect. Eg I did it earlier (same day). That is not correct in formal English.
Not a teacher at all