Use of do vs does

Question

In the sentence

What would a healthy person do?

Why is it correct to use do and not does?

Answer 1

In the sentence "What would a healthy person do?", the use of "do" instead of "does" is correct due to the structure of the sentence. Here’s a breakdown of why:

1. Modal Verb Usage: The sentence contains the modal verb "would." When a modal verb (like "would") is used, the main verb in the sentence (in this case, "do") is not conjugated for tense or subject agreement. Instead, the modal verb dictates the form.

2. Subject-Verb Agreement: In English, the verb "do" would be conjugated as "does" for the third person singular (he/she/it) in simple present tense sentences. However, when a modal verb is used, "do" remains in its base form regardless of the subject.

So, in the sentence "What would a healthy person do?", the structure is:

- Modal verb: "would"

- Subject: "a healthy person"

- Base form of the main verb: "do"

This construction is correct and natural in English. If we were to rephrase the sentence in a simple present tense form without the modal verb, it would be: "What does a healthy person do?" Here, "does" is used to agree with the third person singular subject "a healthy person."

Answer 2

In the sentence "What would a healthy person do?", the correct use is do because would is a modal verb. Modal verbs are verbs that express possibilities, requests, abilities, opinions etc. etc. When you use a modal verb in a sentence, use the infinitive form of the verb.

For example, let's use the sentence "I can read", which is the modal verb and which is the infinitive form of the verb? The modal verb is can because it expresses ability or possibility. The infinitive form of the verb is read because you're talking about your ability to read.

If you were to replace the word do with does in your sentence, "What would a healthy person does?" grammatically speaking, that does not make sense. If you want to use the word does instead of do, you can rewrite the sentence as "What does a healthy person do?" So, you replace the word would with does because now, it's a present tense question as you have two verbs that are present tense.

Another rule to consider when deciding whether to use do or does in a sentence. You use the word do with plural subjects and pronouns such as I and you while you use the word does with singular subjects and pronouns such as he and she. While this rule may not apply to the context of the sentence that you have in your question, it is nonetheless an important rule to remember.