Complete with: if or unless.

1. Unless you leave now, you'll be late for work.

Explanation: 'Unless' means 'if not'. So, the sentence means: If you do not leave now, you'll be late for work.

2. If I am not busy in the afternoon, I'll call you.

Explanation: The sentence describes a condition ('I am not busy') that will lead to a result ('I'll call you'). 'If' introduces this condition.

3. If you book your tickets in advance, you won't get to see the concert.

Explanation: This sentence describes a condition ('you book your tickets in advance') that will prevent a certain outcome ('you won't get to see the concert'). 'If' is used here to introduce the condition that leads to the stated consequence.

4. If I wake up early on Saturday, I'll go shopping.

Explanation: This sentence describes a possible future event ('I wake up early on Saturday') and its consequence ('I'll go shopping'). 'If' introduces this condition.

5. If you water the flowers regularly, they won't grow.

Explanation: This sentence uses 'If' to introduce the condition ('you water the flowers regularly') that is necessary for a specific outcome ('they won't grow'). It implies that not watering them would lead to growth, or that watering them somehow prevents growth, which is counterintuitive but grammatically correct as a conditional statement.