The words in yellow tell you that there are grammatical mistakes in those words. You will also see the corrected suggestions displayed. Now it’s your choice whether you want to replace the yellow colored words with the suggested corrections or not. You must remember that Grammar Check Free checks your work from a purely grammar aspect and offers suggestions. It will also check spelling mistakes, as quite often grammar mistakes are caused by spelling mistakes. With a word or words spelt correctly in a sentence the grammar error should not appear. However, if the grammar error is being caused due to some other reason it will be displayed again and the reason for the error will be pointed out. The decision to replace the original with the suggestion has to be yours. It will under no circumstances replace any sentence or words in the original document that you have submitted for checking. It will only point out errors and suggest changes. Some writers follow their own writing rules and choose to ignore suggestions. But that is the user’s discretion.