A grammar checker is software or a program feature found in a word processor and is used to find grammatical errors. That is to say, it checks for improper sentence structure and word usage (e.g., their instead of there), poorly placed or unnecessary punctuation, and other more esoteric errors. An example of a software program that includes its own grammar checker is Microsoft Word. MS Word underlines grammar errors with a green squiggly underline as shown in the picture.
Tip: Press the F7 function key to start the spelling and grammar checker in Microsoft Word and most word processors.