I have asked eighteen questions so far. But Spanish SE is not accepting any more questions from my account. So, I need to improve my existing questions, so that I can answer more questions. So, how can I improve my existing questions?
The post linked by mdewey is quite informative. There's nothing I can add to it but I would like nonetheless to copy here some paragraphs.
Automatic bans never expire or "time out." This means that you cannot simply wait for a certain amount of time. If you do not take action, you will never be allowed to freely post again. The only way for the ban to be lifted is by contributing positively to the site in other ways.
It seems that you have no other choice but to try to improve your questions. Note the emphasis on "improve". Deleting your questions will not remove the ban, you need to improve them. Many of your questions, as noted before, try to ask about things that are just mere coincidences. As an example, the last one (which I see you have deleted) tried to ask about why many nouns have synonyms of both masculine and femenine genders. That's just a matter of probability as half of the nouns are masculine in Spanish and the other half is feminine. Or are they? Maybe you can undelete and rephrase the question to "what is the percentage of masculine nouns in Spanish and what is the percentage of feminine ones?", so you can then calculate the probabily of a specific synonym to be of a gender or another.
Another example: you asked about why "baca" and "baja" have opposite meanings but are so similar (just another coincidence). Maybe you could change the question just to "what is the etymology of 'baca' and when did it start to be used in Spanish?" and ask a similar question for "bajo, a" and just reach your own conclusions. Just try to rephrase your questions to avoid asking for coincidences.
Moderators cannot lift the ban.
This means that the ban removal depends exclusively on you, we moderators cannot do anything about this.
Do note that rewriting an existing question into a substantially different question is frowned upon. Your edits should not negate or invalidate existing answers. The fixed post should fundamentally be about the same issue and topic, and the existing answers should still apply to it.
In the case the question you are going to edit has no answers yet then I suppose it's safe to change it substantially. I'll turn a blind eye in those cases, but be careful with the questions that already have answers.
One last thing, if you want, flag any post you edit for moderator attention and I'll write a comment asking people to remove the negative votes if the rephrased question deserves it. I hope you will get that annoying ban removed. Good luck!