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My mother could play tennis very well when she was young.
→when以下が過去なのでcanをcouldにします
I thought it might rain.
→時制の一致によってmayをmightにします
I had to go to school one hour earlier yesterday.
→mustに過去形はないのでhad toにします
He was determined that he wouldn't attend the party.
→時制の一致によって、won'tをwouldn'tにします
助動詞や時制の一致の項目を確かめてみてください(^-^)V
とても分かりやすい説明ありがとうございます(´;ω;`)
英語もっと頑張ります!m(*_ _)m