Grammatical rule for using has vs. does

Question

Last week a young adult from Haiti, enrolled in our ESL class, asked the following question when presented with the following:

Tea has caffeine, and so does coffee.

His question was: "Why can't you use the word "has" in place of "does"?

I had two responses. First of all, I don't believe that a comma is needed because the second clause is not independent. Secondly, I prefer not to use a word twice in a sentence. I know the first use of "has" indicates past, present, and future, but, beyond that, I'm not certain what to tell him.

Greg T.

Answer 1

Your sentence is a perfect example of a dummy verb. A dummy verb is a type of auxiliary verb used in place of another verb. Often used to do exactly what you said: avoid saying the same word twice.

It's often used in answers also:

• Q: "Do you take this man to be..."
• A1: "I take him." Would be awkward, so
• A2: "I do." Do is standing in for take.

See this page and scroll down to the dummy verb section.

When does is used with have it does form the emphatic tense, as portcall said. But when the main verb is left out I would say it no longer functions as emphatic tense. The emphatic auxiliary verb is being used as a placeholder for the main verb

Answer 2

@G. Tomevi: Taken alone, "So does coffee." is incomplete, yes. But since it follows a sentence which contains THE complete thought, it becomes complete. Sentences of this construction should normally follow a sentence which contains the complete thought, or part of the whole context. In this case, "Tea has caffeine." does the job of providing the complete thought.

@Roaring Fish: This is the part of the original post which contains "does."

"...so does coffee."

Not only is this clause emphatic but it is also inverted (the verb does precedes the subject coffee). Moreover, this second clause follows an independent clause (Tea has caffeine) from which it takes meaning to complete the thought.

To construct its non-emphatic form, I have to 'borrow' the thought from the first clause to make it meaningful, and I also have to construct the sentence in normal order (not inverted). This is the result: "Coffee has caffeine too."

In the non-emphatic or normal form, a sentence uses only the main verb (in this case, "has"). In the emphatic form, on the other hand, the verb is in the following format: auxiliary verb + main verb (does + have).

If used together with the first clause it would read this way: "Tea has caffeine, and coffee has caffeine too."

Notice the redundant use of has caffeine. I firmly believe that the reason why the writer of the sentence decided to go for inversion and use the emphatic form in constructing the second clause, as well use so in place of too, is to avoid redundancy.

Answer 3

Your sentence is correct, but it's also fine to say

tea has caffeine, but so has coffee.

Now, this isn't a educated grammatical answer, but I think 'do' is working a little like a pronoun. You know the way you can simplify a specific noun with 'it', say, when it's clear what you're referring to?

Coffee has caffeine. it is bitter.

Notice we're avoiding repeating the noun by using the pronoun as a placeholder.

I think 'do' is acting in a similar way. We know we're talking about 'have'. So you insert the more general 'do' instead of repeating yourself. For 'have' it's not very helpful, but for long verbs?

Concussion grenades discombobulate people, and alcohol discombobulates them too.

Sounds silly. It's less silly to say

Concussion grenades discombobulate people, and alcohol does too.

Note that the 'does' also swallows 'them' out of the sentence.

I have no idea whether this is a known thing or nonsense, but it makes sense to me.

Answer 4

Tea has caffeine, and so does coffee.

His question was: "Why can't you use the word "has" in place of "does"?

The use of 'does' is correct. But you're right, there should be no comma. In fact, if I were to correct the sentence, I would rewrite it as follows: Tea has caffeine. So does coffee.

To better understand why "does" is used instead of "has" (and why it is correct to do so), one needs an understanding of the emphatic form of sentences. Also called emphatic tense or emphatic mood, it is formed with the use of the present or past tense of the auxiliary verb do + base form of the verb. The subject sentence, in emphatic form, goes this way (using my revised version):

Tea does have caffeine. So does coffee [have].

(The second sentence is the inverted form of, "Coffee does have [coffee] too.")

To further illustrate, this time using the past tense:

Simple form: John went to school yesterday too.

Emphatic form: John did go to school yesterday too.

Emphatic, inverted form: So did John.