**Rule**: The **Present Perfect Continuous Tense** is used to emphasize the duration ('all week') of an action that started in the past and is still ongoing. **Correct Usage**: 'have been waiting' perfectly describes the long, continuous period of anticipation for the package. **Incorrect Options**: (a) 'wait' (Simple Present) is a habit. (b) 'am waiting' (Present Continuous) is for now. (d) 'waited' (Simple Past) is a completed action.

**Rule**: This sentence combines a **Simple Present** state with a **Present Perfect Continuous** action that explains it. **Correct Usage**: 'He **is** an expert' describes his current state (Simple Present). The reason for this state is that 'he **has been studying** the subject for over 20 years', showing the long, continuous action leading to his present status (Present Perfect Continuous). **Incorrect Options**: The other combinations create a mismatch in tenses. (a) 'is, studies' uses the wrong tense for the durational action. (c) 'has been, studies' incorrectly uses present perfect for the state. (d) 'was, studied' puts both clauses in the past.

**Rule**: The **Present Perfect Continuous Tense** is used for a habitual action that started at a specific point in the past ('since it first aired') and has continued regularly up to the present. **Correct Usage**: 'have been watching' correctly emphasizes the long, continuous viewership of the show. **Incorrect Options**: (a) 'watch' (Simple Present) is a habit without the historical context. (b) 'am watching' (Present Continuous) is for now. (d) 'watched' (Simple Past) is a completed action.

**Rule**: The **Present Continuous Tense** can be used to describe a temporary, ongoing action or a project that is in progress over a period of time. **Correct Usage**: 'is learning' correctly conveys that the process of learning is currently happening, even if not at this exact second, and it's a temporary situation. **Incorrect Options**: (a) 'learns' (Simple Present) would imply a completed habit. (c) 'has learned' (Present Perfect) would mean the learning process is finished. (d) 'has been learning' would also be possible but 'is learning' is more common for describing a current project or hobby.

**Rule**: The **Present Perfect Continuous Tense** is used to describe an action or state that started in the past and has continued without interruption over a period of time ('all day'). **Correct Usage**: 'has been suffering' emphasizes the continuous, ongoing nature of her discomfort throughout the day. **Incorrect Options**: (a) 'suffers' (Simple Present) would imply she suffers from headaches habitually. (b) 'is suffering' (Present Continuous) focuses only on the present moment. (d) 'suffer' is the base form.

**Rule**: **Stative Verbs**. The verb 'taste' when describing a quality is stative and used in the **Simple Present Tense**. **Correct Usage**: 'tastes' correctly describes the current quality of the soup. The subject 'It' is singular. **Incorrect Options**: (a) 'is tasting' is incorrect as 'taste' is stative here. (c) 'has tasted' is the wrong tense. (d) 'is' would need an adjective, e.g. 'It is a bit bland'.

**Rule**: The **Present Perfect Continuous Tense** is used with 'lately' to describe an activity that has been happening over a recent period and is not yet complete. **Correct Usage**: 'have been looking' correctly describes the ongoing, so-far-unsuccessful search that has been taking place recently. **Incorrect Options**: (a) 'look' (Simple Present) is a habit. (b) 'am looking' (Present Continuous) is for now. (d) 'have looked' focuses on completion.

**Rule**: The **Simple Present Tense** is used to state permanent facts or general truths of nature. **Correct Usage**: The fact that the river flows towards the sea is a permanent geographical truth. The subject 'The river' is singular, so the verb is 'flows'. **Incorrect Options**: (a) 'is flowing' (Present Continuous) describes the action at this moment, which is also true, but simple present is better for a permanent fact. (b) 'has flowed' (Present Perfect) is the wrong tense. (d) 'flow' is for plural subjects.

**Rule**: The **Present Perfect Continuous Tense** is ideal for describing a trend or a continuous action that has been happening over a period of time ('for the past two years') and is still ongoing. **Correct Usage**: 'have been increasing' accurately depicts the continuous upward trend in profits over the specified duration. The subject 'profits' is plural. **Incorrect Options**: (a) 'increase' (Simple Present) would state a general fact, not a trend over time. (b) 'are increasing' (Present Continuous) describes a current trend but doesn't connect it to the past two years as effectively. (d) 'increases' is for a singular subject.

**Rule**: The **Present Perfect Tense** with 'just' is used to describe a thought or realization that has entered one's mind a moment ago. **Correct Usage**: 'has just occurred' correctly expresses the recent dawning of the realization. **Incorrect Options**: (a) 'just occurs' (Simple Present) is less common for a sudden thought. (b) 'is just occurring' is incorrect as 'occur' (in this sense) is stative. (d) 'just occur' is a plural verb.