Somebody who spelled my name wrong thought it mattered.....
I have spent my life telling people about my name because it is spelt Jackalyn in full and you do not get that here in England. In fact I think the three other people with the name are on my facebook, and I have always been Jacky with a y in protest at my many Great Aunts and Uncles who for no apparent reason decided I was Jackie-Ann. Even the Vicar at my Christening tried to tell my parents they could not spell.
Both Autocrit and Grammarly have free options. That will not tell you how to correct anything, but if you are hard up it gives you some idea. On balance, I paid for autocrit. You do need to be sure that any program you use is looking at the kind of English you are writing in. American and English-English are just not the same. American and English spelling are also different at times.
The other thing that is useful is the add- on in Open Office, which I prefer to Word anyway. It gives you instant readability reports and so on which is very useful. I expect there is the same kind of thing in Word.
If you are using the KJV in your text it is best to take that out before you do the analysis as the programs will analyse that as well. Half the time, if I do not do that, then I find the problems the program is picking up is the wording of the KJV. Yesterday the Open Office add on seemed to be telling me the longest and worst sentence was one of my Bible favourites!
I wish there was a programme that looked at book formatting. When I finally get a template right in open office anyone who is on FW can have it for free. (Will work with LULU.)
I wish all these aids had been around and PC's too when I wrote my Anthropology dissertation! I can't think how I wrote all those words by hand, and without any 'helps' at all, apart from a Grammar text book. I am currently feeling very annoyed with someone writing their dissertation and thinking how hard it is. It would have been so easy with all these things to help, and I would not have had to ask the woman who typed it manually to go back and correct all the words she 'corrected' which were right in the first place!