I would like to share a few guidelines I have about how to answer language related questions from learners (which I haven't always followed on this forum). I have answered many such questions as a teacher over the past 9 years or so and for me, anyway, the following rules are very useful.
The most important guideline is to understand the context of the language in the question. If someone asks about a word, get them to show the word in context. If someone asks about a pattern of words (ie "grammar") get the context. The second guideline, and most essential teacher activity, is to think of as many additional analogous examples of the language in context as we can (at least 2 or 3) and write them or say them out loud. This is important because our gift as native speakers is the infinite body of examples we carry in our heads.
The third guideline is to avoid generalising beyond the examples that are now in play (the original and the ones you have supplemented). As native speakers the tendency is to make absolute statements about the language in question because we think we know intuitively about the language we speak. Nothing is further from the truth! Our native language is probably the language we have spent the least amount of time analysing and therefore know the least about.
If we have a created a mini corpus of 3-6 example sentences, make sure any further discussion holds true for those examples. The learner is usually satisfied with that.
Further discussion about the language should prioritise meaning first, form second, and usage last.
Provide the meaning (face to face with a learner you would ideally get them to guess the meaning of the target language from the examples).
If the basic meaning is clear to the learner, there is time, and the learner has indicated that they want it, look for any unusual forms or grammar patterns, eg. the plural of 'child' is 'children' or "stand up" versus "stand out". Again it is important to confine yourself to your mini corpus to avoid misleading statements about the language. If there is a juicy grammar pattern or word form you really want to provide to the learner and is not present in your mini corpus, think of a few more example sentences first and make sure they are analogous to the original corpus, otherwise you risk muddying the waters.
Finally (again if circumstances warrant) point out how different forms correspond to different circumstances of usage such as formality, mode (written or oral), genre etc.
The above is just a template I like to follow in my mind. As circumstances dictate, I will skip parts while keeping the general priorities and sequence in mind. I must say however, that the brainstorming of example analogous sentences is essential and should always be followed explicitly. In fact if I taught just the meaning and the student were just to review the examples I provided, I believe they will have gotten 95% of what I can give them as a teacher.
As a further note, when a student asks such a question, my top priority is to evangelise the approach of collecting examples on flashcards and learning unanalysed "chunks" of language on their own, coupled with massive input.
