1. If you travel to the USA, you can/should/must have a visa. 2. Passengers must/must not/should not smoke in the toilets.

3. The doctor thinks you should/must/may eat less and take more exercises.

4. You shouldn't/don't have to/couldn't laugh at old people.

5. A good advertisement can/may/must be clear and brief.

1. If you travel to the USA, you must have a visa.
Explanation: When traveling to the USA, it is a requirement to have a visa. This is a mandatory rule set by the US government for people entering the country. A visa allows you to legally enter and stay in the USA for a specific purpose, whether it is for tourism, business, or studying. Without a visa, you will not be allowed entry into the country.

2. Passengers must not smoke in the toilets.
Explanation: Smoking in the toilets is strictly prohibited. It is a safety hazard as the smoke can set off alarms and pose a fire risk. Additionally, it is also disrespectful to other passengers who may not want to be exposed to secondhand smoke. Passengers must follow this rule to ensure the safety and comfort of everyone on board.

3. The doctor thinks you should eat less and take more exercises.
Explanation: The doctor is suggesting that you should adjust your diet and engage in more physical activities. This recommendation is based on the doctor's assessment of your health or weight-related concerns. Eating less means consuming fewer calories, which can help with weight loss or maintaining a healthy weight. Taking more exercises refers to being physically active, such as doing regular workouts or engaging in sports. These lifestyle changes are advised by the doctor to promote good health and overall well-being.

4. You shouldn't laugh at old people.
Explanation: Laughing at old people is considered impolite and disrespectful. It is important to treat elderly individuals with respect and kindness, just as we would want to be treated when we are older. Making fun of or laughing at someone based on their age is hurtful and can negatively impact their self-esteem and mental well-being. Instead, it is important to show empathy, understanding, and appreciation for the wisdom and experiences that come with old age.

5. A good advertisement should be clear and brief.
Explanation: An effective advertisement needs to convey its message clearly and concisely. By being clear, it means that the advertisement should be easy to understand and not leave the audience confused about the product, service, or message being promoted. Being brief refers to keeping the advertisement concise and to the point, avoiding unnecessary information or lengthy explanations. This is important because in today's fast-paced world, people have limited attention spans and are often bombarded with numerous advertisements, so it is crucial to capture their attention quickly and effectively.