Different usage of "simple past tense" and "present perfect"

Question

I have a question regarding the usage of "simple past tense" vs "present perfect tense". Sometimes I see they are both possible to talk about a finished action. I know we cannot use "present perfect tense" to talk about a finished action, that have mentioned time frame. But my question is different. Suppose, something happened yesterday but I do not want to mention the time rather I want to describe what happened. In this kind of situation which tense is appropriate?

Here's the context:

• I bought/have bought a mobile. It has a stunning look and takes nice pictures. I am very happy with it.

Please note, that I bought the phone yesterday. I know if I used yesterday in that sentence I had to use "simple past tense". But as I have not mentioned any time frame, Can I use both the tenses? 

Please also note that I just suddenly start a conversation with "I bought a mobile", with no preamble.

Answer 1

Both are about "finished action" in a sense but one is not specific. But, I understand what you mean. It's about the relationship of what you want to say with the present, known as the time of speaking.

It all depends on what you want to say:

1) In your context, "I have bought a mobile phone" just means you are telling us this occurred in the past but when that happened does not matter in the conversation or what you are trying to convey. It merely signals past, at the time you are speaking in the present.

2) The minute you want to be specific by either explicitly informing us of when you actually bought it or even if you don't specifically state it, you use the simple past.

Both are correct. However, both do not have the same relationship to the time of speaking.

If you say: |I bought a mobile phone and I love it| bought implies a specific moment in time even if you do not state it: yesterday, last week, two days ago. That is why sometimes people call it a finished action. It really should be called an action that occurs at a specific moment in time.

have bought just means in the past and when exactly is not relevant to the present time in which I am speaking. This meaning conveyed by ELLers is sometimes quite hard to grasp unless you speak Spanish, for instance.