1. Lan/ must/ finish/ all/ homework/ tonight
Lan must finish all her homeworks tonight.
2. You must not/ walk/ grass/ park
You must not walk on the grass in the park.
3. We/ must/ follow/ all/ signs/ swimming pool
We must follow all signs in the swimming pool.
4. She/ ought to/ help/ mother/ housework
She ought to help her mother do housework.
5. Ba/ can/ swim/ across/ river
Ba can swim across the river.
6. Small children/ can/ learn/ swim/ quickly/ easily
Small children can learn quickly and easily.
7. We/ ought to/ clean/ classroom
We ought to clean our classroom.
8. You/ should/ exercise/ regularly/ keep/ fit/ healthy
You should do excercise regularly to keep fit your healthy. 
9. They/ should/ take part/ sports activities
10. you/ must not/ stay up/ every night