Present perfect

[Hanka]

Hello,

thank you for all your help. Could you please explain in which situations would the following tenses be used:

1) I have decided to study.
I decided to study.

2) I have started running a business.
I started running a business.

Thank you very much.

Hanka

[Humble]

Hello, Hanka,
You could use these in the Pr.Perfect if the info is new for the listener and if no specific time of the action is indicated.
Frankly speaking, sentence 1a) looks odd and incomplete. You know, a lot depends on the context.
A situation for 2a) could be the following: you've just met a former classmate of yours, whom you have not seen for a couple of years and you're telling her all your news.
If you say Last year... then goes 2b).
The PrPerfect is much simpler than the Past Perfect (the more I learn, the less I know but basically all the tenses are easy.
Best wishes

[rewboss]

Generally, if the present is more important than the past, you would be more likely to use the present perfect.

"So... now it's time to make your decision."
"Yes. I have decided to study."
(Connection with the present: The decision is complete and I am going to act on that decision.)

"What did you do after that?"
"Well, I decided to study, so I enrolled at a university..."
(No connection with the present. We are definitely talking about a past event. Perhaps somebody is telling their life story.)

"I have started running a business."
(Connection with the present: The business is now running. Most likely, the business is still very new.)

"I started running a business, but it failed."
(No connection with the present. We are talking about a past event, but it's not important how this affects the present. In fact, because the business has failed [note my use of the present perfect here!] it no longer exists, so that's another reason to use the past simple.

Note that the present perfect is not quite so common in American English. It's used more in British English.