This is a sentence:

John walks.

This is another sentence:

Mary runs.

That’s it. That’s all there is to it.

Two words and a period.

Then why is it so complicated? Why is it so hard to make a sentence look and act like a sentence?

It shouldn’t be… because there are only two parts to a sentence:

the subject (who or what) + the verb (is or does) + a period (usually)

Everything else adds time or conditions or information or whatever. (By the way, the verb is sometimes referred to as the predicate, which actually means everything that isn’t part of the subject. So:

subject (who or what) + predicate (is or does) + a period (usually)

Take a look at The Tale of Mr. Morton to see how it all works.