A "complementary infinitive" is an infinitive used with a verb whose meaning is not felt to be complete: "you ought" or "they dare" by themselves are not specific about either your duty or their daring. Thus, even in English we use an infinitive to complete the meaning of verbs like this: "we ought to go shopping this weekend," or "you are able to play tennis," or even "they dared to attack the martians." In Latin, the same thing happens, often with verbs that will require an infintive in English as well: examples include debeo (I ought), possum (I am able), or audeo (I dare).
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