Affirmative: She has bought a mobile phone.
Interrogative: Has she bought a mobile phone?
Negative: She hasn’t bought a mobile phone.
1. For actions which happened at an unstated time in the past. The exact time is not mentioned because it is not important. We put more emphasis on the action.
* Kim has bought a new house. (When did she buy it? We don’t mention the exact time because it is not important. What is important is the fact that she’s got a new house).