Answer:
The rule which states that if a verb is in past tense, then its subject should be in past tense too and if it's in present tense, the subject should also be in present tense.
“I was amazed at his equanimity in the face of danger.”
The verb "was" is in past tense so is the subject "amazed".
Explanation:
A simple subject-verb agreement definition says that the subject of the sentence and the verb of the sentence must be in agreement in number/tense.
If the sentence is written like this
“I am amaze at his equanimity in the face of danger.”
If the verb "was" is replaced by "am" and the subject "amazed" is replaced by "amaze", the sentence would become incorrect as it disregards the subject verb agreement.