The "someone" may (=possibly) have missed to point out the context in which "may" is used as "do", but that has a lot more to do with culture than language:
In English phrases like "You may want to sit down." are a way of avoiding a direct commands like "Do sit down." which may (=possibly) come across as impolite.
The straight translation / literal meaning of "may" remains the same ("Perhaps you want to sit down?") but the underlying message is a straight request or order.
So how to distinguish between those? It's a question of context. Who is the speaker? What is the topic? Is there a request or demand in a friendly packaging? Or is the speaker simply expressing a possibility or probability? In the former case you can remove all "may" and all verbs like "want" or "consider" to get the core message.
It may start to rain soon. -> possible, no request
You may want to finish this task first. -> order. Read: "Finish this task first."
There is also the may indicating permission ("May I have this seat?"), but that was not part of your question.