The present perfect tense is used to describe actions or events that happened at an unspecified time in the past but have relevance or effect in the present. It can also describe actions that started in the past and continue up to the present.

Structure
The structure of the present perfect tense is:

Subject + has/have + past participle (third form of the verb)

- "Has" is used with singular subjects (he, she, it).
- "Have" is used with plural subjects (I, you, we, they).

Usage of present perfect tense - 

1. To talk about experiences or actions at an unspecified time in the past
   - Example: She has visited Paris.
     - (The exact time of the visit isn’t specified; we only know it happened at some point.)

2. To describe actions that started in the past and continue to the present
   - Example: I have worked here for five years.
     - (This implies that I started working here five years ago and still work here now.)

3. To describe recent actions with present effects
   - Example: They have just finished their homework.
     - (This suggests the action was completed recently, and the effect is still relevant.)

4. To announce or describe changes over time
   - Example: My English has improved.
     - (This indicates progress or change from the past to the present.)

Examples of present perfect tense - 
- I have lost my keys. (I don’t have them now.)
- She has lived in New York since 2010. (She still lives there.)
- We have just had lunch.
- They have completed the project.

Time Expressions Often Used
Common time expressions with the present perfect tense include:
- ever, never (for life experiences): Have you ever been to Japan?
- just, already, yet (for recent events): I have just finished my work.
- for + a period of time: for two years, for a while
- since + a specific starting point: since January, since last week
- lately, recently

 Key Points
- Does not specify when an action happened but shows its relevance to the present.
- Emphasizes the connection between the past and the present.
  
The present perfect tense is ideal for discussing experiences, unfinished actions, and recent events that have an impact on the present moment.