To write with no grammar mistakes - you should read a lot.
If you have met "definitely" word thousands times in your books how you can forget spelling when need to write it?

Russians know their native language also really bad. Lots write with idiotic school mistakes. Many people sometimes don't know meanings of used words. Almost nobody can use punctuation properly.
I guess this is the reason of your "in Russian, many words are not capitalized". Because of bloggers who are too lazy to poke Shift button in their keyboards.