1. Why doesn't he practice law?
    Why isn't he practice law?

  2. What is it?
    What does it?

  3. Why doesn't he come?
    Why isn't he come?

  4. It doesn't matter who you marry.
    It isn't matter who you marry.

  5. He doesn't care a thing about me.
    He isn't care a thing about me.

  6. She doesn't realize it.
    She isn't realize it.

When should I use is, and when does?
Are these sentences correct?

4 Answers 4


In all your examples only one sentence is correct every time. "Is" is a main verb there. That means we translate it and it is the only verb in the sentence. For example:

What is it?

There is no other verb besides is, and this sentence is correct.

"Do" is not a main verb, it is used to make negatives and questions. There is always another verb in the sentence, which we translate. All the other sentences are examples of that; the verbs are: practice (first example), come (third example), matter (fourth example), care (fifth example), realize (sixth example).

  • The issue is not whether there is a "main" verb or not ,but rather what form of the associated verb (with is or does) is correct. Even the second instance using "does" could be correct in context if the preceding text or statements were setting up some form of action as the topic under discussion.
    – DWin
    Commented Apr 13, 2013 at 18:21
  • @DWin:It is about main verb and auxiliary. These are pairs which normally appear in A1 level exercise, which is done after learners have covered the present simple of "to be" (which does not take an auxiliary) and begin to study the present simple of other verbs and encounter the auxiliary "do". Later in the same level they also study "to be" as an auxiliary in the Present Continuous but since all the verbs in the examples are in the infinitive, the question seems to ask about the Present simple tense.
    – fluffy
    Commented Apr 14, 2013 at 11:42
  • So this forum is about some artificial (albeit possibly widely used) system that does not necessarily ratify idiomatic constructions as "correct"?
    – DWin
    Commented Apr 14, 2013 at 17:22
  • @DWin: we seem to be talking about the same thing, using different words. What the forum is about sounds like a topic for ELL meta; the most important thing here is that we can answer the question in the best way possible so people will get the help they asked for. We seem to have already achieved it here. Sorry, I would have written this in a personal message if it was possible and asked what you mean when you talk about idiomatic constructions. I am not sure where I should ask the question.
    – fluffy
    Commented Apr 14, 2013 at 20:21
  • I think you are implying that respondents on this forum should not be ordinary native speakers but rather should be teachers (or perhaps other students) who are familiar with a particular course system. I'm a literate native speaker whose formal English training and its associated terminology was 40 years ago. There were no "main verbs" back then. I dispute the classification of "What does it?" as "incorrect" and see it as being "incorrect" only if the local standard has been codified with a system that the forum has chosen.
    – DWin
    Commented Apr 14, 2013 at 22:00

It has to do with form. "Why doesn't he practice law?" is correct. But the correct is form is "Why isn't he practicing law?"

"Why doesn't he come?" is correct. Again, the is form is "Why isn't he coming?"

"What does it matter whom you marry?" is correct. If I were to use the "is" form, it would be "What is it mattering whom you marry?" (Except that there is no such word as mattering, so no is form is correct here.)

On the other hand, "What is it?" is correct; "What does it?" is not.

English is confusing.

  • 1
    Also "What does it do?" would be correct, wouldn't it? What is does and what it is are two different things, of course.
    – Stephen
    Commented Apr 13, 2013 at 18:05

In most of the comparisons the 'does' version is more correct because of the association with a present tense form of another verb. The 'is' versions (with the exception of the second pair) fail because in English you pair "is" with an "-ing" version of the associated verb, forming the present participle. The present particle is also called a "gerund".

Why isn't he practice law?    Instead use: Why isn't he practicing law?

Why isn't he come?      Instead:  Why isn't he coming?

It isn't matter who you marry.     Instead:  It isn't mattering who you marry.

He isn't care a thing about me.     Instead: He isn't caring a thing about me.

She isn't realize it.       Instead: She isn't realizing it.

(Admittedly the sentence 'It isn't mattering who you marry.' sounds rather tortured to this English speaker, but it think it is grammatically correct.)

  • "It isn't mattering who you marry." would certainly never be said by a native speaker of English.
    – Matt
    Commented Apr 15, 2013 at 7:52
  • As I said. It sounds "tortured" and I think its because the verb "matter" doesn't really have the sort of activity connotation that lends itself to the "ongoing" implication conveyed by the present participle construction. This is like some uses of the verb "is"; "matter" usually assigns an all-time value attribute to an event.
    – DWin
    Commented Apr 15, 2013 at 16:24

Except for the second couple of sentences, all the other sentences have something in common: They negate a verb.

If you are negating the verb in a question:

  • If the verb is be, you replace are with aren't, and is with isn't
  • If the verb is another one, you generally replace do/does (the auxiliary verb) with don't/doesn't

If you are negating the verb in a sentence that isn't a question:

  • If the verb is be, you replace are with aren't, and is with isn't
  • If the verb is another one, you generally add do/does (does is for the third singular person) before the verb

In your examples, practice, come, marry, care, realize are all verbs, and they are not be.

"What is it?" is correct if for example you are asking what the object you are looking is. "What does it?" would eventually be "What does it do?" and it is asking what the purpose of an object is.
When you ask "What is it?" people normally say what that object is; if they think you didn't understand what it is, they could also explain what the purpose of that object is. Although, the two questions have different meanings.

Notice that matter is also a noun; as such, it is used in sentences like the following ones:

It is a matter for the police.

It is not a matter of not being able to do something; it is just a matter of knowing the right people.

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