Only One!

There can be only one! is the tag line from the 1986 film Highlander. Flash synopsis: through the ages, immortals fight each other and can only die when beheaded; the last one standing wins the Prize – whatever that is (no spoilers here).

So in the end, there can be only one. I use that tagline as a mnemonic, as it illustrates an essential point of grammar: there can be only one subject Form. Why? It's a matter of efficiency.

If there are two tasks and you're alone, you'll have to do every thing yourself. But if there's someone to help you, you delegate one of the tasks for efficiency. That's what happens with verb forms, and that's why English doesn't allow He doesn't goes, nor She didn't went, let alone *Bob will SMILEs.

There can be only one! (Connor MacLeod - the Highlander)

One or the other

In sentences, verbs must show the subject form and indicate the main verb. If there's only one verb, it must perform both tasks single-handedly. As soon as there are two verb forms, they split the workload for efficiency. That's how it works.

You may occasionally find examples of double subject forms, but that doesn't make it official. Sometimes, it's a regional or social dialect that adds amazing flavour to the language; sometimes, it's nonsense. In standard English grammar, it's just not done.

So as a rule, there can be only one!

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