What is the past tense of pain

Well, pain is a noun so there's no past tense of it. That's like saying what's the past tense of "happy" or what's the past tense of "a watermelon". Instead, you'd need a verb to go with that noun and then put that verb in past tense form.

Past Tense -  I felt pain

Present Tense - I am feeling pain

Future Tense - I will feel pain

In the example above "feel" is the verb. You can then turn that verb into past tense, present tense, or future tense. Pain is the noun that goes with that verb.

what the first or next guy says

a, he promoted disarmament