Catching typos and getting a second opinion before submitting anything is what the Review tools are for.
A red squiggly underline flags a possible spelling issue; a blue or green one flags possible grammar. Right-click the underlined word or phrase for suggested fixes.
Shown live at the bottom-left of the window — useful the moment an assignment has a minimum or maximum word limit.
Track Changes (Review tab) marks every edit so someone else can see exactly what changed and by whom. Comments let you leave a note attached to specific text without editing the text itself.
Word silently fixes common typos as you type via AutoCorrect — "teh" becomes "the" automatically. This occasionally "corrects" something on purpose (an abbreviation, a name) into something wrong. File → Options → Proofing → AutoCorrect Options shows and controls exactly what it changes.