According to the dictionary
definitions (e.g. in Merriam-Webster)
, "should" is the past of "shall" and
"might" is the past of "may":
But are these modal verbs really used
as such? I know they are frequently
used with other meanings (as confirmed
by their other definitions in the
dictionary), but my question is if
they are really used with their
original meaning of past tense for
"shall" and "may".
No, they are not used that way, Bruno. That's very perceptive of you. In modern English, the modal verbs are tenseless. They have neither past or present tense.
I give two examples below. Do the
present and past versions have the
exact same meaning (except for the
tense, of course)?
1) MAY / MIGHT
Present: When he is at school, he may
not go to the bathroom without asking
for permission.
Past: When he was at school, he might
not go to the bathroom without asking
for permission.
2) SHALL / SHOULD
Present: He thinks that they shall go
the restaurant.
Past: He thought that they should go
the restaurant.
These are not examples of past and present tense. Your examples have nothing to do with tense. The backshifting that occurs in examples like these [may --> might; will --> would; can --> could] have nothing to do with an actual tense change, eg.
I'm going to jump over the line.
I jumped over the line.
The only reason for this backshifting in your examples is to denote that the speech is not a direct quote. As an example, lets use my example above, about the 'jumping'.
A: I'm going to jump over the line.
B: [to C] What did he say, C?
C: He said [that] he was going to jump over the line.
What C said doesn't constitute a finished action, which would trigger the use of true past tense usage. Again, the only reason for the backshift to the past tense FORM "was", is to tell the listener,
"I'm not vouching that this is exactly what he said, I'm only giving you a general description of the event".
The use of 'said' does denote a true past tense. What he said is finished, past, hence the use of a real past tense, as opposed to the use of a past tense FORM.
We know that this isn't a past time event because even in a semantic sense, it doesn't describe a past event. The event hasn't happened yet. A is still standing there waiting his turn to jump over the line.
We also know it isn't a real past event because native speakers have a choice as to whether we backshift or not.
C could also say [note 'could' describes a future potential event, NOT a past one]
C: He said [that] he is going to jump over the line.
Notice the use of "is" rather than "was". Native speakers often maintain the present tense FORM when the event is current, happening right now.
So to review. Modal verbs are tenseless verb forms. Their jobs are to add emotive senses to speech and writing and in the SPECIAL case of REPORTED SPEECH, they also are used to mark speech as reported.