<(Or they've convinced themselves they do by hearing the same thing repeated over and over again in the textbooks!)>
Not at all. Why do you think there are two forms? Are they synonyms? And if so, why?
Modals follow the same pattern as any other verb in English regarding, Time, Relationship and Possibility or Likelihood (my caps). What are traditionally called the present forms are used for close time, close relationships and close possibility/likelihood. What are traditionally called the past forms are used for distance in time, distance in relationships and distance regarding possibility/likelihood.
Traditionally, it is said that "might" is more polite than "may", but what does polite mean there? It means that with uses of "might" we can imply, express or create more social distance than with "may".
EG Might/May I help you? (Interesting to note that the version with "might" is more often found accompanied by "sir" than "may" is.)
The same with:
Will it rain tomorrow?
It might/may.
There, "might" signal less likelihood of rain in the mind of the speaker
Non-modally:
In the clothes shop:
Do you want to try that dress on?
Did you want to try that dress on?
Why do you think there are two forms there?
Other modals:
Can you help me, please? (Closeness, or less distance, in social relationship/closeness in possibility)
Could you help me please? (Distance, or less closeness, in social relationship/distance in possibility. More tentative.)
It has been that way for centuries. Long before EFL textbooks as we know them today were invented.
![Tongue Tied [:S]](/emoticons/emotion-7.gif)