I've found some sentences that seem odd for me such as, "She is married" instead of "She was married" or "This transaction is approved" instead of "This transaction was approved"
I want to know which one is correct? (from those example) and Are there any rules for "is/am/are" and "was/were" in this case?
In this case, married and approved are used as an adjective, which is also a subject complement. As such, it gives information about the subject (She, The transaction).
The use of the past tense would imply, for example that "She is not married anymore", and consequently single again.
married and approved is more the status of the subject after the action of marrying and approving, as the description of the action itself. If it is still true, then the present should be used, otherwise, use the past.
In your examples using "to be" means it's something that has happened but is still true, because it's a change of state from one state to another. If you're referring to the transition itself you would use "to be" as long as the state is unchanged.
Examples: "He is married". (The state of being married) "He was married in the late summer 10 years ago". (The celebration of the transition from unmarried to married) "He was married" (More ambiguous, but most likely he's no longer alive, his partner is no longer alive, or he is divorced, etc. Unless you specify a specific time it most likely means the state is no longer true)
