Microsoft Word for word processing is one of the most central and basic programs used by students and teachers. It can be used in a very limited way, but also has some excellent advanced features for those willing to get to know their way around.
We suggest using full/app Word when you are working on anything other than a simple document. If you'll be inserting tables, graphs, pictures, videos, icons... use full Word. You can dictate in full Word mode and track changes if you are reviewing a document. There are also features (like synonyms) available in full/app Word but not Word online/browser. When in full/app Word, make sure that you create the document in OneDrive, or that you save the document to OneDrive before you start writing so it will Autosave. (In the picture above, Autosave is off because the document has not been saved there.) Of course you can also share and collaborate... and much more using Word.
To learn get some tips including writing equations, check out Microsoft help and training.