I don't think it is necessary to be a teacher to answer grammar questions on forums, but I do think it is better if one is a native speaker. In general, native speakers don't make grammatical mistakes. There are some non-native speakers who are as good, or maybe even better than the average native speaker, but I personally can't tell how good a non-native speaker is. So I prefer to ask native speakers, just to be sure.
If you are a native speaker, don't worry about not being a teacher. If you aren't a native speaker, then maybe we'll both learn something.
Which is correct:
1-I am thinking about the man yesterday.
2-I am thinking about the man from yesterday.
(A man that I saw yesterday.)
I agree with F. :D
Let's try some context: :D
Curtis: Who should we hire for the position?
Rod: I'm not sure. We've seen at least 12 people this week alone.
Sue: What about the applicant with marketing experience?
Rod: Do you mean the person we interviewed after lunch?
Sue: No. I'm thinking about the man from this morning. :D