Both has and had are possible depending on the perspectives of the speaker. " which implies time passed.
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DearYolandaMr. Ward has been telling us about the quiz since last week, but I didn’t understand what he has/had been saying.... has been telling us about the quiz since last week, but I haven't understood what he has been saying.
DearYolanda"I just found out that we are having a math quiz tomorrow. " means "I" didn't understand Mr. Ward's words for a week, until just now I finally figured out what he has been talking about. Is i