What are the key differences between the KJV and the NASB?

Answer 1

The NASB is a better translation for many reasons.

More ancient manuscripts to provide more accurate renderings. 1 John 5:7,8

We now know the ancient Hebrew and Greek better than the scholars did in 1611.

We speak a different 'English' than those who spoke in 1611.

The KJV is basically the Latin Vulgate translated into 1611 English. The oldest Hebrew and Greek manuscript the translators of the KJV had is dated back to 800 CE.

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Answer 2

Accuracy.

The NASB adds original syntax and subtracts archaic language.

Unlike the KJV, it's translated from the original texts.