I've downloaded the program but can't get it to open on Windows 10
Do you have Java 8 installed? If so, what exactly happens when your try to start LT?
No, I don't have Java 8. It opens to various files but not the software.
You'll need to install Java 9 first from the link above, LT cannot work without it.
I've downloaded Java but Word doc opens in LT as unreadable jumble
Word doc opens if I copy and paste the text in the window! This is OK for short pieces but for several pages of tex,t I shouldn't I be able to open the Word doc in the window?
Word format is not supported. Other formats aren't supported either, only plain text. You could try https://github.com/jaumeortola/languagetool-msword10-addin#english instead, but it has several limitations.
You could also download LibreOffice and the LT extension for LibreOffice. AFAIK LibreOffice Writer supports MS Word format.
Thanks - I've given up on LT and found another checker that's easier to download and use.