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Had heard is cast in the Past Perfect tense, to make it "deeper in the past" compared towith the Simple Past construction "was not surprising". There is no plural-singular distinction for the had verb in Past Perfect.

Compare:

"To those of us who have heard the principal the news seems grim", said John.

From the standpoint of John, the time when he spoke was present. This is why John used the Present Perfect construction have heard, and since those is plural, he used have.

John said that to those of them who had heard the principal the news seemed grim.

Now we are reporting what John said. We backshift seem to seemed (Simple Past), and backshift have heard even deeper in the past, making it had heard (Past Perfect).

Had heard is cast in the Past Perfect tense, to make it "deeper in the past" compared to the Simple Past construction "was not surprising". There is no plural-singular distinction for the had verb in Past Perfect.

Compare:

"To those of us who have heard the principal the news seems grim", said John.

From the standpoint of John, the time when he spoke was present. This is why John used the Present Perfect construction have heard, and since those is plural, he used have.

John said that to those of them who had heard the principal the news seemed grim.

Now we are reporting what John said. We backshift seem to seemed (Simple Past), and backshift have heard even deeper in the past, making it had heard.

Had heard is cast in the Past Perfect tense, to make it "deeper in the past" compared with the Simple Past construction "was not surprising". There is no plural-singular distinction for the had verb in Past Perfect.

Compare:

"To those of us who have heard the principal the news seems grim", said John.

From the standpoint of John, the time when he spoke was present. This is why John used the Present Perfect construction have heard, and since those is plural, he used have.

John said that to those of them who had heard the principal the news seemed grim.

Now we are reporting what John said. We backshift seem to seemed (Simple Past), and backshift have heard even deeper in the past, making it had heard (Past Perfect).

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source | link

Had heard is cast in the Past Perfect tense, to make it "deeper in the past" compared to the Simple Past construction "was not surprising". There is no plural-singular distinction for the had verb in Past Perfect.

Compare:

"To those of us who have heard the principal the news seems grim", said John.

From the standpoint of John, the time when he spoke was present. This is why John used the Present Perfect construction have heard, and since those is plural, he used have.

John said that to those of them who had heard the principal the news seemed grim.

Now we are reporting what John said. We backshift seem to seemed (Simple Past), and backshift have heard even deeper in the past, making it had heard.