How can you explain English grammar?!

Why would anyone want to study grammar? Just take a look at the word “take” in the following sentences.

Keith takes on the bad guys.
Watch Keith take on the bad guys.
Watch as Keith takes on the bad guys.

If somebody were to try to explain the grammar and why the word is “takes” in the first sentence but changes to “take” in the second sentence and back to “takes” in the last sentences, you would have too much to think about.

I think the best thing to do is “get a feel” for the language. That of course involves getting lots of input.