Ayuki_Holmes
It seems to me that you're just looking for an easy way out. Veronica has given you some awesome resources on how to recognize tense shifts and hence be able to fix it, but you don't seem to want to put in the time and effort to learn it yourselves first. If those articles still end up whooshing over your head -as per your claim- then you probably should go back to learning basic rules of tenses again. And there's nothing wrong with that.
Some authors here are lucky to get themselves a free editor, but when it boils down to improving your writing skills, it's as simple as putting in the time and effort on it.
Quoting Lilliny:
Lilliny Zinni for free? uh if you wanna improve your grammar you have to read published and edited works in English. There's no shortcut to fluency