Explain the rules for forming the present perfect tense.

The present perfect tense is used to describe an action that started in the past and has a connection to the present. It is formed by using the auxiliary verb "have" or "has" followed by the past participle of the main verb.

The rules for forming the present perfect tense are as follows:

1. Affirmative Sentences:
- For regular verbs, add "-ed" to the base form of the verb to form the past participle. For example: "I have worked."
- For irregular verbs, use the past participle form of the verb. For example: "I have eaten."

2. Negative Sentences:
- Use the auxiliary verb "have" or "has" followed by "not" and the past participle of the main verb. For example: "I have not seen him."

3. Interrogative Sentences:
- Begin the sentence with the auxiliary verb "have" or "has" followed by the subject, and then the past participle of the main verb. For example: "Have you finished your homework?"

4. Contractions:
- The contraction "haven't" is used for "have not," and "hasn't" is used for "has not." For example: "She hasn't arrived yet."

5. Time Expressions:
- The present perfect tense is often used with time expressions such as "already," "yet," "just," "recently," "ever," "never," "so far," "up to now," etc. For example: "I have already finished my work."

6. Use of "For" and "Since":
- The present perfect tense is often used with the prepositions "for" and "since" to indicate the duration of an action. "For" is used to express a period of time, while "since" is used to indicate a specific point in time. For example: "I have lived in this city for five years" or "I have known her since 2010."

It is important to note that the present perfect tense is not used with specific time expressions that refer to the past, such as "yesterday," "last week," or "two days ago." In such cases, the simple past tense is used instead.