On questions about English grammar how can somebody choose a best answer if they didn't understand the grammar originally?

The words 'idiocy' and 'stupidity' come to mind.

Native speakers have known the language so long that we cannot remember the actual source of our knowledge. If we gave our source as "I learned this from my English teacher in 1965" it would not be helpful to questioners today. Most answers can be verified on google.

By verifying the source, if you are fortunate to have people who provided sources for their answers, that you can then look at yourself.

I actually saw an answer on here that posted a source that actually refuted the person's answer (they didn't read it very carefully).

Otherwise, either choose by the quality of the answer (even if you can't verify it), or by the first of the most promising answers.