Let's say "Bob", a hypothetical ELL, comes to this site and posts this question.
Which of this sentences is grammatically correct?
How is you feel today?
How are you feel today?
(Ignore whether or not the question should be closed, since this is just an example.)
Now, it's fairly obvious that the question is about is/are, but if this was a real question, I would also point out that it should be feeling instead of feel even though that is not the point of the question. That's an easy example, and I do this pretty often with questions of this type.
But should I also correct the grammatical mistakes in the actual question itself? It would probably be edited out fairly quickly, but should it be pointed out to the OP? On one hand, he is here to learn English, and you learn English (or any language) by making mistakes, and learning from them. On the other hand, I can imagine it would be frustrating if you're having trouble learning something, and you come here to ask a question about X, and everybody here is nitpicking on how you did Y wrong.