**Rule**: The **Simple Present Tense** is used to state a long-term, ongoing fact or trend. **Correct Usage**: 'continues' correctly describes the persistent, long-term trend of population growth. The subject 'The world population' is singular. 'is continuing' is also possible but simple present is excellent for stating a persistent fact. **Incorrect Options**: (b) 'is continuing' (Present Continuous) also works to show a current trend. (c) 'has continued' (Present Perfect) focuses on the past continuation. (d) 'continue' is a plural verb.

**Rule**: The **Present Perfect Tense** is used to describe a recent action whose result is important in the present. **Correct Usage**: 'has learned' indicates that the learning process has happened and the result is that the intern is now a valuable team member. The focus is on the completed achievement. **Incorrect Options**: (a) 'learns' (Simple Present) would be a general statement about the intern's ability. (b) 'is learning' (Present Continuous) implies the process is still ongoing. (c) 'has been learning' would emphasize the duration of the learning process rather than the result.

**Rule**: The **Present Continuous Tense** with 'always' is used to express annoyance at a repeated, irritating habit. **Correct Usage**: 'are always playing' effectively conveys the speaker's frustration with the neighbors' frequent and annoying behavior. **Incorrect Options**: Using the Simple Present ('always play') would state the fact neutrally without the emotional overtone of annoyance. (a), (c), and (d) are grammatically incorrect in this specific structure of complaint.

**Rule**: The **Present Perfect Tense** with 'just' is used to talk about a very recent past action with a result in the present. **Correct Usage**: 'have just heard' indicates the action of hearing the news happened a moment ago, and the result is the new information about the storm. **Incorrect Options**: (b) 'am, hearing' is incorrect as 'hear' is a stative verb of sense and not typically used in continuous form. (c) 'do, hear' is the wrong structure. (d) 'have, been hearing' would imply hearing the news repeatedly over a period, which is unlikely.

**Rule**: The **Present Perfect Tense** is used to describe an action or state that started in the past and has continued over a long duration ('for over fifty years') up to the present. **Correct Usage**: 'have maintained' correctly describes the continuous state of peaceful relations over the specified period. The subject 'The two countries' is plural. **Incorrect Options**: (a) 'maintain' (Simple Present) lacks the historical duration. (b) 'are maintaining' (Present Continuous) focuses on the present only. (d) 'maintains' is a singular verb.

**Rule**: **Stative Verbs**. The verb 'belong' indicates possession or a state of being a member of something. It is a stative verb and is not used in continuous tenses. **Correct Usage**: 'belongs' correctly states the permanent fact of ownership. The subject 'That building' is singular. **Incorrect Options**: (a) 'is belonging' is grammatically incorrect. (c) 'has belonged' could be used with 'since' or 'for' to show duration, but simple present is better for a simple statement of fact. (d) 'belong' is a plural verb.

**Rule**: The **Present Perfect Continuous Tense** is used to talk about an ongoing, long-term project or intention that has been in progress for a period of time ('for a while now'). **Correct Usage**: 'has been planning' correctly conveys the continuous nature of this strategic thinking over a period that leads up to the present. **Incorrect Options**: (a) 'plans' (Simple Present) is a general habit. (b) 'is planning' (Present Continuous) doesn't incorporate the duration from the past. (d) 'planned' (Simple Past) is a completed action.

**Rule**: The **Present Continuous Tense** used with 'always' expresses annoyance or criticism about a frequently repeated, irritating habit. The structure is **am/is/are + always + verb-ing**. **Correct Usage**: 'is always leaving' conveys the speaker's frustration with the sister's repetitive and annoying habit. **Incorrect Options**: (b) 'does, leave' is an incorrect structure. (c) 'has, left' (Present Perfect) refers to a completed action. (d) 'leaves' (Simple Present) would state the habit neutrally without the added emotion of annoyance.

**Rule**: The **Simple Present Tense** is used to state a fact that is currently true. 'Meet' in this context means 'to satisfy or fulfill'. **Correct Usage**: 'meets' correctly states the current fact that she satisfies all the requirements. 'Has met' would also be correct, emphasizing the completion of fulfilling the requirements. **Incorrect Options**: (a) 'is meeting' (Present Continuous) is less common for this meaning. (b) 'has met' (Present Perfect) is also a very strong correct answer. (d) 'meet' is a plural verb.

**Rule**: **Stative Verbs**. When 'see' means 'understand', it is a stative verb and is not used in the continuous form. It's used in the **Simple Present Tense**. **Correct Usage**: 'I see what you mean' is a common idiom for 'I understand what you mean'. **Incorrect Options**: (b) 'am seeing' is used for the physical act of looking at something or for dating someone. (c) 'have seen' refers to past visual experience. (d) 'sees' is for a third-person singular subject.