Do

As an auxiliary, do is essentially a "dummy"; that is, it doesn't generally affect the meaning. It is used to form questions and negations when no other auxiliary is present: "I don't want to do it." It is also sometimes used for emphasis: "I do understand your concern, but I don't think that will happen." Also, do sometimes acts as a pro-verb: "I enjoy it, I really do [enjoy it], but I'm not good at it." (Other auxiliaries do this as well: "I can do it, I really can [do it], it just takes me longer"; but it bears particular note that in the case of do, it is often used as a pro-verb when it would be absent if the verb were present.) Because it does not affect the meaning of its verb, not all grammarians acknowledge do as a modal auxiliary. In a sense, it indicates a lack of modal auxiliary. (Do is also different in that it has a distinct third-person singular form, does, and in that its past tense, did, is used exactly as a past tense, not as a more general remote form.)