Microsoft Word autocorrects many mistyped words. It underlines in red words that it doesn't recognize. This is helpful.

Microsoft Word doesn't recognize ligatures (two letters joined, usually fi or fl). E.g., it thinks that office is misspelled.

Microsoft Word underlines grammatical errors in green. If you don't understand why your grammar checker flagged a sentence, click on Tools…Spelling and Grammar and it will tell you. If you don't understand its explanation, look it up in a grammar book (e.g., The Chicago Manual Of Style).

The Grammarian grammar checker (made by Casady & Greene, http://www.casadyg.com) is more powerful than Microsoft Word’s built-in grammar checker. If your writing is worth money, it’s worth $19.95.