Q&A: Is there a rule for determining the vowels in Hebrew conjugation (present, past, future and passive)?

Yes, there is: it’s determined by the binyan (construction) of the verb.
There are seven binyanim, each with its characteristic pattern of vowels. Their names reflect the vowel pattern in the past tense (3rd person singular, masculine)—thus:

The appropriate binyan of a verb is determined by whether it is a simple verb, or one that refers to an action to or on something else; or a manipulation of an object or a person; or a continuous or repeating operation—or the passive corollary of any the above. (For more information on that, see my post on How do I know if a verb in hebrew is pa’al type, pi’el type, or other?)