Even some professional writers, ahem, aren't always sure what a prepositional phrase looks like. So it's hard to expect a kid to keep all that stuff straight. For them (and us!), several sites come to the rescue. Our favorite is Exploring English, a straightforward, kid-friendly guide to all things grammatical. Here you'll find diagrams showing the parts of speech, a rundown of types of sentences (remember compound complex sentences?), and all those other picky things English teachers insist we all memorize at least once.

A less simplified, but more comprehensive, rundown of the same material is atHyperGrammar, an online grammar textbook. It's pretty dry, but it makes up for that by having lots of examples.