This question asks why we translated the Greek word κύριος used in Matthew 8:8 and 21 as "Sir" in Matthew 8:8 but as "Lord" in Matthew 8:21.
Why is the same Greek word being translated as both 'Sir' and 'Lord' in these two different verses?
Because the context of the usage of κύριος is different in each verse. The ISV v1.4.6 of Matthew 8:8 reads:
8 The centurion replied, “Sir,[1] I am not worthy to have you come under my roof. ...”
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[1] 8:8 Or Lordand the ISV v1.4.6 of Matthew 8:21 reads:
21 Then another of his disciples said to him, “Lord,[1] first let me go and bury my father.”
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[1] 8:21 Or Sir
Why not translate the word κύριος the same in each place?
Because doing so would make the ISV an inferior rendering of the Greek contextual usage of κύριος.
Is it because there two different speakers, one was a centurion and the other one was a disciple?
Yes. You figured it out... Do take a look at the footnotes in verses 8 and 21. Each acknowledges an opposite possibility. We've done this because the text does not say in verse 8 that the centurion was a disciple, so we've rendered the word κύριος as "Sir" in verse 8. The text does say the person in verse 21 was a disciple, so we've rendered the word κύριος as "Lord" in verse 21.
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