To fill in the gaps with a modal verb, let's analyze each sentence and determine the appropriate modal verb to use:
You fall asleep at the wheel.
- Should you fall asleep at the wheel.
You feed the cat, he's not hungry.
- You need to feed the cat, he's not hungry.
Can you help me move the furniture?
- Will you help me move the furniture?
If you're still waiting, then leave.
- If you're still waiting, then you can leave.
The fire spread quickly, but everyone managed to escape.
- The fire spread quickly, but everyone was able to escape.
The fish swims, but does not fly.
- The fish can swim, but does not fly.
My grandfather was very smart, he spoke four languages.
- My grandfather was very smart, he could speak four languages.
I'm hungry. "Don't worry, I'll make you a sandwich."
- I'm hungry. "Don't worry, I will make you a sandwich."
You're shouting, I can hear you.
- You're shouting, I can hear you.
She has a fever, she won't go for a walk today, she'll stay in bed.
- She has a fever, she won't go for a walk today, she'll stay in bed.
- Mary keeps crying, she has a problem.
- Mary keeps crying, she must have a problem.
- I think you'll take the train, it's faster than the bus.
- I think you should take the train, it's faster than the bus.
- "Am I inviting you to the next dance?"
- "Shall" I invite you to the next dance?
- Don't wait for me. I'm staying in town for the night.
- Don't wait for me. I am staying in town for the night.
- The milkman added water to the milk.
- The milkman must have added water to the milk.
- The boy fell into the river, but we saved him.
- The boy fell into the river, but we managed to save him.
- Can I borrow your bike tomorrow?
- May I borrow your bike tomorrow?
- Dad, you smoke so much, it's bad for you.
- Dad, you smoke so much, it's bad to you.
- Sir, do I have a suggestion? I agree.
- Sir, might I have a suggestion? I agree.
- We're not sure about tomorrow, but we're going to the beach.
- We're not sure about tomorrow, but we will go to the beach.
- I arrived home at eleven in the evening because I work late.
- I arrived home at eleven in the evening because I have to work late.
- She hates getting up early.
- She hates getting up early.
- If you go to the city, you will definitely go shopping.
- If you go to the city, you will definitely go shopping.
- Take an umbrella with you, it won't rain.
- Take an umbrella with you, it won't rain.
- Do you think you'll tell your teacher what happened?
- Do you think you will tell your teacher what happened?
- Children leave school early tomorrow.
- Children will leave school early tomorrow.
- You drive so fast that we are not late.
- You drive so fast that we won't be late.
- Are we going to the mountains? - No, let's go for a swim.
- Are we going to the mountains? - No, let's shall go for a swim.
- You can only smoke in the dining room, you smoke in this room.
- You should only smoke in the dining room, you smoke in this room.
- You leave medicines in places where children can get them.
- You should not leave medicines in places where children can get them.
These are the correct modal verbs to fill in the gaps for each sentence.