I am not a newbie at SE and SFF.SE. I know how and what to post here. But still, I am getting too many downvotes. Why? How to reduce that?
Let's have a look at the negative-scoring posts from the first page of your questions.
Now, the downvote tooltip is
You have 3 duplicates, 2 thoroughly covered in canon, obviously those didn't "show any research effort." And I suspect some downvoting was for "not useful." And I think people give less slack to users who have been around a while than they might give a newcomer.
Finally, I hate to say it but it's likely some of your downvotes were because of your English. I know it isn't your native language, and I'm sure you try hard, but sometimes it can be hard to understand what you're asking. Users really should try to fix your posts rather than just downvote for this, but it doesn't always happen.
I'm posting this as a 2nd answer because it's quite different from my first answer and it only occurred to me after re-reading your question for the fourth or fifth time. And this may be the root of the issue. The problem is I'm not good at sugar coating and this answer assumes that since you have asked for a critique of your posts that you're willing to examine what you're doing.
Look at two sentences in your question:
They're almost contradictory and that's the heart of it. I know you've been here a good while, but downvotes are the community's way of saying one's posts are less than stellar. So the very fact that you're getting so many downvotes is an indication you may not know how to post here. My other answer prompted you with some questions.
It seems to me if the community is voicing the opinion that your questions are not good, then you may feel you know know how to post, but you are missing something important somewhere along the line. While we have tried to say what it is, there doesn't seem to be one clear answer yet.
I would suggest taking the downvotes as an indication you are doing something wrong-- and you've started that process by asking for help here, so you're doing something good. But take it farther. Look at your posts, then start comparing them to posts with low votes ore multiple downvotes. Look at unpopular posts and see what makes them unpopular. Then compare them to your posts for common elements.
Most would say look at popular and liked posts and do more like that, but I find, in writing, I learn more from looking at bad examples and knowing what not to do than just reading what's good.
While I've learned a lot from reading Shakespeare, if I want to use all the rules he used, I'd be writing just like he did. But I can't tell you how much I learned from watching bad writing in Lost in Space and other shows I liked as a kid but now know are poorly written.
If you look at a great writer and try to be like him or her, you're just imitating, but if you look at bad examples and learn what NOT to do, that still leaves things open for you to find your own style that's good.
It's been a while since I've had time to watch questions and answers closely, but a few general thoughts would include watching trends. Are some types of your questions getting more downvotes than others? And what is different about your questions, when compared to other people's questions? I don't mean in terms of subject matter, but in how you are asking them and what you say?
Some of them you were just unlucky to encounter crabby users. E.g. people who downvote duplicate questions despite the fact that there doesn't appear to be an obvious way to have known it was a duplicate when asking. That's quite rude, IMHO but you can't escape it. Vote to close, flag, but don't mark it as a BAD question. (this is different if googling for question's title brings the SE dupe as first hit - THEN it's a bad Q).
Some of them are downvoted - again, IMHO, not very fairly, because "the answer is obvious if you watched the episode/movie". Again, I think that's both rude, AND wrong. The question's audience is not only those who have perfect memory or obsessively watched some episode 10x times. It could be someone who watched it ways back and doesn't remember details, or didn't watch at all. As I have previously commented to someone who was being an a-hole in the comments on a similar vein, many of THAT user's questions seemed way trivial and idiotic to people very familiar with the works. "Trivial" and "obvious" are too subbjective to be used as a valid reason to DV.