Ex. 1. fill in the gaps with if or when and a verb in the present tense, as in the examples. 1. we might go for a walk tomorrow. if we go, we will take the dog with us. 2. the guests will arrive soon. when they arrive, we will greet them at the door. 3. i am going to phone sam in a minute. … him, i want you to leave the room. 4. i might visit pamela tomorrow. … her, i will buy her a present. 5. the bus comes at eight o'clock. …, we will all get on it. 6. she might invite us to her party. … us, we will go. 7. the film will start soon. …, i will record it. 8. mark may lend me some money. … some money, i will buy that jacket.

Sure! Here is a detailed and step-by-step solution to the exercise:

1. We might go for a walk tomorrow. If we go, we will take the dog with us.
- If we go for a walk tomorrow, we will take the dog with us.

2. The guests will arrive soon. When they arrive, we will greet them at the door.
- When the guests arrive, we will greet them at the door.

3. I am going to phone Sam in a minute. If I phone him, I want you to leave the room.
- If I phone Sam, I want you to leave the room.

4. I might visit Pamela tomorrow. If I visit her, I will buy her a present.
- If I visit Pamela tomorrow, I will buy her a present.

5. The bus comes at eight o'clock. When it comes, we will all get on it.
- When the bus comes at eight o'clock, we will all get on it.

6. She might invite us to her party. If she invites us, we will go.
- If she invites us to her party, we will go.

7. The film will start soon. When it starts, I will record it.
- When the film starts, I will record it.

8. Mark may lend me some money. If he lends me some money, I will buy that jacket.
- If Mark lends me some money, I will buy that jacket.

In this exercise, we use "if" to talk about a condition or possibility in the future, and "when" to talk about something that is certain or expected to happen in the future. The verb in the present tense after "if" or "when" indicates what will happen if the condition is met or when the event occurs. It is important to note that "if" suggests a possibility or uncertainty, while "when" suggests a certainty.