**Rule**: The **Present Perfect Tense** is used to describe a completed action or a set of actions within an unfinished time period ('this morning'). **Correct Usage**: 'have answered' indicates that the task of replying to all the morning's emails is now finished. **Incorrect Options**: (a) 'answer' (Simple Present) is a habit. (b) 'am answering' (Present Continuous) means the task is in progress. (d) 'have been answering' would emphasize the continuous process of answering, not the completion of the whole batch.

**Rule**: The **Present Perfect Continuous Tense** is used to ask about a mental activity that has been ongoing for a duration ('for the last ten minutes') and has a visible present result ('You've been smiling'). **Correct Usage**: 'What have you been thinking about?' correctly inquires about the cause of the prolonged smiling. **Incorrect Options**: (a) 'do, think' asks for an opinion. (b) 'are, thinking' is for now. (d) 'have, thought' is for a completed thought process.

**Rule**: The **Present Perfect Continuous Tense** is used to describe a long, continuous action that started in the past and continues to the present, especially when it has a visible present result ('She looks exhausted'). **Correct Usage**: 'has been working' emphasizes the duration ('for the past year') and the tiring nature of the continuous action that causes her current state. **Incorrect Options**: (a) 'works' (Simple Present) is a habit. (b) 'is working' (Present Continuous) doesn't connect to the duration. (d) 'has worked' (Present Perfect) focuses on the experience, not the ongoing, tiring effort.

**Rule**: The **Present Perfect Continuous Tense** is used for a continuous action that has been happening over a duration of time ('for three days') and is leading to a present or imminent action. **Correct Usage**: 'has been observing' emphasizes the continuous surveillance that has led to the current point where an arrest is imminent. **Incorrect Options**: (a) 'observes' (Simple Present) is a habit. (b) 'is observing' (Present Continuous) is for now. (d) 'has observed' (Present Perfect) focuses on the completion of the observation.

**Rule**: **Dynamic vs. Stative Verbs**. While 'taste' is often a stative verb (describing quality, e.g., 'The soup tastes good'), it can be a **dynamic verb** when it refers to the action of testing something. **Correct Usage**: Here, 'is tasting' refers to the chef's action of sampling the food at this moment. The word 'now' confirms this is an action in progress. **Incorrect Options**: (a) 'tastes' would be used to describe the quality of the food, not the action. (c) 'has tasted' refers to a completed action. (d) 'taste' is the base form.

**Rule**: The **Present Perfect Tense** in the negative with 'yet' is used to show that an expected action has not happened up to the present moment. **Correct Usage**: 'hasn't given' correctly states that he has not expressed his opinion in the time leading up to now. **Incorrect Options**: (a) 'doesn't give' (Simple Present) is a habit. (b) 'isn't giving' (Present Continuous) is for now. (d) 'didn't give' (Simple Past) requires a specific past time frame.

**Rule**: While 'hope' is a stative verb, the **Present Perfect Continuous Tense** ('have been hoping') is sometimes used informally to emphasize the continuous, ongoing nature of a desire over a period of time ('all week'). **Correct Usage**: 'have been hoping' stresses the continuous and perhaps frustrating state of hope over the past week. 'have hoped' would be the more grammatically standard choice. **Incorrect Options**: (a) 'hope' (Simple Present) is for the present. (b) 'are hoping' is also possible but less connected to the duration. (d) 'hopes' is a singular verb.

**Rule**: The **Simple Present Tense** is used to state general truths or scientific facts. **Correct Usage**: The fact that water boils at 100 degrees Celsius is a universal truth. The subject 'Water' is a third-person singular (uncountable) noun, so the verb takes an '-s'. Thus, 'boils' is correct. **Incorrect Options**: (a) 'is boiling' (Present Continuous) would mean the water is boiling right now. (c) 'has boiled' (Present Perfect) implies a recently completed action. (d) 'boil' is used with plural subjects.

**Rule**: The **Present Continuous Tense** can be used to talk about current analyses or predictions that are being made. **Correct Usage**: 'are predicting' suggests this is the current consensus or ongoing discussion among economists. The subject 'The economists' is plural. Simple Present ('predict') is also possible to state their conclusion as a fact. **Incorrect Options**: (a) 'predict' is also a valid option. (c) 'have predicted' (Present Perfect) announces the prediction as recent news. (d) 'predicts' is a singular verb.

**Rule**: The **Present Perfect Tense** is used with stative verbs (like live, know, be) to describe a state that started in the past and continues to the present, especially with 'for' and 'since'. The Present Perfect Continuous ('have been living') is also possible and emphasizes the continuous nature. **Correct Usage**: 'have lived' is correct because the action of living started in the past (1995) and continues now. The subject 'My parents' is plural. **Incorrect Options**: (a) 'live' (Simple Present) doesn't convey the duration from the past. (b) 'are living' (Present Continuous) is usually for temporary situations. (c) 'has lived' is for singular subjects.