We could go to the zoo if you like. 
We couldn't see the elephants because there were too many people. 
Could you see the tigers? 

You could try learning English. 
You couldn't learn English, you're too stupid. 
Could you learn English in school? 

I could run very fast when I was a child. 
I couldn't ride a bike until I was 18. 
Could you speak Spanish when you were young? 

They could be English people. 
They couldn't be Irish people, they have an English accent. 
Could they be American? 

You guys could eat Mexican food tonight. 
We couldn't eat robot food, we're people. 
Could you eat Mexican robot food? 

You must do your homework. 
You mustn't ask others to do your homework. 
Must I do your homework for you? 

(we don't often use 'must' in questions, we'd say 'do I have to?' but since you want sentences with must, I'll use must.) 

We must Speak English in our English class. 
We mustn't speak Spanish in our English class. 
Must we speak Italian in our English class? 

I must go to the shops to buy dinner for my family. 
I mustn't feed them KFC every day. 
Must they eat so much every day? 

You must wear school uniform to school. 
You mustn't wear a hat inside. 
Must we wear a tie? 

I must remember to shower in the morning. 
I mustn't forget to shower in the morning. 
Must I really shower every day?