Find and correct the mistakes in the sentences a. Brian returned home because he had forget his phone. b. Helen started crying because she never seen the snow. c. I took a taxi because I hadn't missed the bus. d. Once I had finish studying, I watched TV. e. The team celebrated because they have won the cup.

Here are the corrected sentences with explanations: a. Incorrect: Brian returned home because he had forget his phone. Corrected: Brian returned home because he **had forgotten** his phone. Explanation: After "had", we need to use the past participle form of the verb, which is "forgotten". b. Incorrect: Helen started crying because she never seen the snow. Corrected: Helen started crying because she **had never seen** the snow. Explanation: To express something that happened before the starting of crying, we use the past perfect tense ("had" + past participle). Also, the correct past participle of "see" is "seen". c. Correct: I took a taxi because I hadn't missed the bus. Explanation: This sentence is already correct. It uses the past perfect tense correctly to indicate that the action of missing the bus did not occur, so the taxi was taken. d. Incorrect: Once I had finish studying, I watched TV. Corrected: Once I **had finished** studying, I watched TV. Explanation: After "had", we need to use the past participle form of the verb, which is "finished". e. Incorrect: The team celebrated because they have won the cup. Corrected: The team celebrated because they **had won** the cup. Explanation: Since the winning of the cup happened before the celebration, we use the past perfect tense "had won".