The properties of "do", its distribution and meaning, are discussed. "Do" is one of the most common error-zones for Indian learners of English. Two analyses of "do" which account for the distribution and meaning of this element are presented. According to the transformational approach, "do" is introduced in sentence structures by a late transformational rule. According to the phrase structure approach, "do" is inserted at the level of the initial structure itself just like any other lexical item. The phrase structure approach has an advantage over the transformational approach because it has some explanatory power which the transformational approach does not have. The most significant effect that the transformational approach makes clear is the fact that in English "do" acts as a dummy element to hold the place. This is necessary in English because it is a language where the subject-place (except in some special cases) cannot be empty. The phrase structure approach accounts for the modal-like behavior of "do.""Do" can be replaced by another apt modal such as "can" or "will." It functions as a modal in several other aspects as well. The diverse properties of "do" can provide insight about the properties of the language to the teacher of English. (SW)