I've written three answers. Two of the three have one downvote each and no upvotes. Could you please take a look at them and tell me how I can improve these two answers:
With the second question you link, you may notice that I already had voted to close, with a comment explaining why, some time before you chose to answer it. I wasn't the one who downvoted your answer, but it's entirely possible that it was meant to admonish you for ignoring the rules and attempting to answer a question of very low quality.
This happens to all of us. I have often been downvoted for answering questions that have later been closed, mostly because I thought the question was valid. I don't take these downvotes personally, since it's not about the quality of my answer. In any case, there's a certain amount of randomness to the voting process. You never really know which questions will take off and which will languish in obscurity (although after a while you kind of get a feel for which ones have potential).
One thought on your answer, particularly this:
according to my instincts in native American English
I try not to rely on my familiarity with English but rather on solid grammar principles, and often corroborating examples from credible sources. A good answer should stand on its own merits rather than on the fact the person is a native speaker -- after all, there is far from complete consensus among the native speakers here.
Plus there are hundreds or thousands of English dialects. If you know your American South, for example, you know that something like:
Them that there done that
can be perfectly valid in the right context, even though it would give an English teacher an aneurysm, and certainly isn't something you want to teach to English learners.
Point is, don't worry about defending your instincts. Write what you feel is the correct syntax, substantiate where possible, and if others agree with you they'll confirm with their votes.
Other people may have some suggestions for the content of your answers, but after a quick glance, I would say the biggest issue with those answers is the quality of the questions they are attempting to answer.
We’ve all experienced the drive-by downvote and usually those votes are offset by upvotes if we put some effort into our answer. When the question we’re answering isn’t well-asked, or is so specific that it’s not very useful to other learners, not as many people view it, which means the answers don’t get voted on much either.
Some people try to discourage others from answering low quality questions by downvoting answers, even though there is nothing wrong with the content of the answer. That practice is against the Stack Exchange guideline asking us to vote based on the content of a post, but it does still happen.
I put together a post listing searches to help find questions that need attention. Some of those searches might help you find better quality questions to answer.