After modal verbs of speculation and deduction in the present, we use an infinitive without to.
After modal verbs of speculation and deduction in the past, we use the infinitive of the verb have without to followed by the past participle of the main verb.
We use must when we are 90% sure that something is true.
We use may, might, could, may not, mightn't when there is a 50% possibility that something is (or is not) true.
We use can't when we are 90% sure that something is not true, but we do not use these modal verbs at all when we are 100% sure.
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After modal verbs of speculation and deduction in the present, we use an infinitive without to.
After modal verbs of speculation and deduction in the past, we use the infinitive of the verb have without to followed by the past participle of the main verb.
We use must when we are 90% sure that something is true.
We use may, might, could, may not, mightn't when there is a 50% possibility that something is (or is not) true.
We use can't when we are 90% sure that something is not true, but we do not use these modal verbs at all when we are 100% sure.