If you have Office 365 Pro Plus and you have updated it, you will notice that Word improved the grammar check functionality. Here are some examples.
Notice the blue double underline.
This indicates a Grammar suggestion.
Earlier, Grammar suggestions were shown as green wavy lines.
Right click on the blue double underline to see the suggestion (Add a comma) and the corrected version (Actually, ). Click on the suggestion to finish the correction.
Another example shows red dotted line. This is NOT a grammatical mistake but another issue – too many words used.
Right click shows the problem and the solution. Word is suggesting that you use a concise rather than verbose representation.
Notice the Read Aloud button. The See More option starts regular Spelling and Grammar check.
I will write a detailed article on Word improved grammar check very soon.