Explain the rules for forming the present perfect continuous tense.

The present perfect continuous tense is used to describe an action that started in the past, is still ongoing in the present, and is expected to continue in the future. It is formed by using the present tense of the auxiliary verb "have" (in its present tense form "have" or "has"), followed by the past participle of the main verb, and then the present participle (-ing form) of the main verb.

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

1. Subject + have/has + been + verb-ing: The subject of the sentence is followed by the present tense of "have" (have for plural subjects and I/you/we/they, has for singular subjects and he/she/it), then the auxiliary verb "been," and finally the present participle (-ing form) of the main verb.

For example:
- I have been studying for three hours.
- She has been working on her project all day.
- They have been playing football since morning.

2. Use of "have" or "has": "Have" is used for plural subjects and for the pronouns I/you/we/they, while "has" is used for singular subjects and for the pronouns he/she/it.

For example:
- We have been waiting for the bus.
- He has been practicing the piano.

3. Use of "been": The auxiliary verb "been" is used after "have" or "has" to indicate the ongoing nature of the action.

For example:
- They have been traveling around the world.
- She has been studying English for two years.

4. Use of the present participle (-ing form): The main verb is used in its present participle form (-ing form) to indicate the continuous aspect of the action.

For example:
- We have been watching a movie.
- He has been playing basketball.

It is important to note that the present perfect continuous tense is often used to emphasize the duration or length of time an action has been happening. It is commonly used with time expressions such as "for" (to indicate the duration) and "since" (to indicate the starting point of the action).

Overall, the present perfect continuous tense is formed by combining the present tense of "have" with "been," and then adding the present participle (-ing form) of the main verb. This tense is used to describe ongoing actions that started in the past, continue in the present, and are expected to continue in the future.