This site undermines its own intent of providing a friendly and accessible introduction to calculus through poor and erroneous grammar. An editor could make it very useful. Some examples:

“OK, but how does calculus models change?” error: subject-verb number agreement. Try “model”.

“The fundamental idea of calculus is to study change by studying ‘instantaneous’ change, by which we mean changes over tiny intervals of time.”

So, a change is a changes? Stay with the singular: “by which we mean a change over a tiny interval of time.”

“It turns out that such [tiny] changes tend to be lots simpler than changes over finite intervals of time.”

Now we have a logical problem. Isn’t the set of tiny intervals of time a subset of finite intervals of time? In ordinary usage, “tiny” is finite, surely; a tiny thing is not infinite. So how can a tiny-time change be simpler than a finite-time change, when it is itself a finite-time change?

Errors of this sort lost my faith early on. I hope they’re corrected, because the promise of the article is appealing.