1. Unless you work hard, you... the exams.

B. will fail is the correct answer. The conditional sentence structure "Unless... you will..." indicates a future consequence.
2. Mr. Minh used to... a lot when he was young.

C. smoked is the correct answer. The past tense "used to" is used to describe past habits or states.
3. Minh drew that picture. No one helped him. He drew it...

A. himself is the correct answer. "Himself" is the reflexive pronoun used to indicate that Minh did the action alone.
4. Hung is... to ride a motorbike to school.

A. not old enough is the correct answer. The structure "not... enough" is used to express a lack of ability or qualification.
5. We have learned English for... nearly three years.

B. for is the correct answer. "For" is used to indicate a duration of time.
6. After a month, Hoa got used to... in her new school.

B. studying is the correct answer. "Get used to" is followed by the gerund form of the verb.
7. She told me... up too late.

B. not to stay is the correct answer. The structure "tell someone not to do something" is used to give a negative command.
8. Her son was absent from school... his illness last Tuesday.

A. because of is the correct answer. "Because of" is used to express a reason for something.
9. That child... to go at once.

B. had better is the correct answer. "Had better" is used to give advice or a strong suggestion.
10. The front yard is... to play soccer in.

C. too small is the correct answer. "Too... to" is used to express that something is not possible due to a certain quality.
11. My brother feels... again after his operation.

C. strong is the correct answer. "Strong" is an adjective describing a physical state.
12. He doesn't enjoy... basketball.

D. playing is the correct answer. "Enjoy" is followed by the gerund form of the verb.
13. This form must have the... of the writer.

B. signature is the correct answer. "Signature" refers to the written name of a person.
14. Jane and Jack stood in front of the mirror and looked at...

D. themselves is the correct answer. "Themselves" is the reflexive pronoun used to refer to both Jane and Jack.
15. Susan is the... of the three girls.

B. prettiest is the correct answer. "Prettiest" is the superlative form of the adjective "pretty," used to compare three or more people or things.
16. Is... the beginning of the day called morning or evening?

A. Is is the correct answer. "Is" is used to ask a yes/no question about a singular noun.
17. Nam... familiar with getting up early in winter two years ago.

A. was is the correct answer. The past tense "was" is used to describe a state in the past.
18. The school yard was... with students.

C. crowded is the correct answer. "Crowded" is an adjective describing a place with many people.
19. These are new born... of our farm.

B. calves is the correct answer. "Calves" is the plural form of "calf," which refers to a young cow.
20. My father always drives... He is a... driver.

D. carefully - careful is the correct answer. "Carefully" is an adverb modifying the verb "drives," and "careful" is an adjective describing the driver.