This is a layman's explanation, because I am a layman.

In english, simple sentences usually have the order subject, verb, object. So for example, "I play baseball". This is called SVO order.

however, there is an invisible part - the helping verb. It is present in your mind, although it isn't always expressed.

  • I play baseball.
  • He plays baseball.
  • He must play baseball.
  • He can play baseball.
  • He should play baseball.
  • He ought to play baseball.
  • He may play baseball.
  • He will play baseball.
  • He's gonna play baseball.
The helping verb is what's between "He" and "play". It adds something to the mood of the sentence - it could be obligation from society, moral judgment, permission, potential, or future. You wouldn't be the first person to think that this is a weird place to put future tense.

Is there really a helping verb in every sentence?

Usually, when the subject of a sentence is he, she, or it, you must add an -s to the verb, but for any other subjects you don't need to change the verb at all. But in the sentences above, why did this "S" disappear in the sentences with helping verbs? Remember, the subject is "He", so you usually need an -s here.

It's because this "S" is a special null form of the helping verb: when the subject is he, she, or it, the helping verb is expressed as -s. But when a non-null helping verb is used, something like "can", "may", etc, that irregular form never gets accessed and the S is not necessary.

Here is more evidence that there is something hidden in a sentence like "He plays baseball". Look what happens when you change these sentences into questions:

  • I play baseball. Do I play baseball?
  • He plays baseball. Does he play baseball?
  • He must play baseball. Must he play baseball?
  • He can play baseball. Can he play baseball?
  • He should play baseball. Should he play baseball?
  • He ought to play baseball. Ought he play baseball?
  • He may play baseball. May he play baseball?
  • He will play baseball. Will he play baseball?
  • He's gonna play baseball. Is he gonna play baseball?
Look at the first two sentences - a new word has appeared! Where did it come from? The usual rule for how to form questions in english is: bring the helping verb to the front. But if a sentence doesn't even have a helping verb, what do you do? Well, you use "Do". But, when the subject is "He", you use "Does"!

So unlike a regular helping verb, when you use "Do", you still use that -s at the end of the verb! ("Does he...") What does this mean? It means that you are not adding a new helping verb at all, because that would stop you from needing the -s. You are just changing the order! So, there was already a "Do" there.

And I expect you already knew this! Because any normal sentence with an invisible helping verb actually does have one there - "I play baseball" is similar to "I do play baseball", and "He plays baseball" is like "He does play baseball".

So, english sentences are not just subject and verb - there is also always a helping verb!

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