Well, that's not quite the way to put it. Those paired modals do not really mean 'Present' and 'Past', respectively. Or, if they **do**, then _must_ has to be considered to be Past, with a Present form that's missing.

Here's how it works. In English, the modal auxiliary verbs have been formed historically from present and preterite stems -- that's where the 'Present' and 'Past' stories come from; **but** all the inflected forms based on them have been lost. So English modal auxiliaries are units, uninflectable and fixed in form, each with its own set of idiomatic, irregular, illogical, unpredictable, and constantly changing set of grammar and usage rules.

There is one context in which some of these pairs do function in time reference, e.g,

 - *When I was young, I **could** do 100 pushups; now I **can** only do 99.*

But not all of them, and not usually; what could this sentence even mean?

 - *_When I was young, I **should** do 100 pushups; now I **shall** only do 99._

Plus [most of the actual uses of modals][1] refer to the present or future, not the past. 

And, to top it off, _must_ comes from the **Past** root of the modal, not the Present. German, which didn't lose the inflected forms of its modals, has _muss_ as the present tense form and _musste_ as the past.

 - *Er muss schlafen.* 'He must/has to sleep'
 - *Er musste schlafen.* 'He had to sleep'

Nobody knows why English didn't keep the present form of _must_, but it didn't. However, it doesn't matter in the long run, because `Present/Past` is the least of the differences between modals. [There's also the distinctions][2] between `Possible` and `Necessary`, `Epistemic` and `Deontic`, and `Auxiliary` and `Periphrastic`, among others. Modals are complicated.

  [1]: http://www-personal.umich.edu/~jlawler/Modality.pdf
  [2]: http://www.umich.edu/~jlawler/aue/modals.html