情态动词是英语中常见的一类助动词，如can、could、may、might、must、shall、should、will、would等。它们和动词原形（不定式）构成短语，表达一种推测、可能性、义务、建议或意愿等含义。
当情态动词和不定式结合使用时，需要遵守以下规则：
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不定式一般不带to。但是，在某些情况下，如need、dare、ought to等后面的不定式需要加上to。 例如：He needs to study English hard. She didn't dare to ask him for help. You ought to listen to your parents' advice.
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情态动词后面的不定式要用动词原形。不需要变化成第三人称单数或进行时态。 例如：He can speak fluent English. They must finish the project on time. She will help us with the homework.
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在疑问句和否定句中，情态动词放在主语之前，后面的不定式不带to。 例如：Can you help me with my homework? She can't afford to buy a new car. Should we go to the party tonight?
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当情态动词和have结合使用时，have要变成动词的过去分词形式had。 例如：You could have told me earlier. He should have studied harder for the exam.
情态动词加不定式是英语学习中的一个重点和难点，需要我们多加练习和注意。通过不断的积累和运用，我们可以更好地理解和运用情态动词，提高自己的英语水平。
