The difference between simple past tense and past principal verbs is how complex the phrase is (in other words, how many words are used to express the verb). An example of a past tense verb would be "She doodled on the paper", while the past principal version of that would be "She had doodled on the paper", or something like that. Simple past tense uses one word while principal uses two words to express the same thing.
In your example for Part A, "suppressed" is one word. This means the answer is "past", since "ed" is still used but only one word describes the action. Similarly, for Part B, the sentence would use one word, which is "sat". It's past tense but only one word.
Lastly, for your third question, the phrase for past principal would be "had talked", as it uses "ed" and two words.
