Question

What was likely the main reason for the decline in knowledge of Latin after the fall of the Roman Empire?

Rural people now had less time to study Latin because they needed to work their land.
Europe became more separated by languages, such as Spanish and French.
Latin evolved into German and English, making it less important to know Latin.
Warrior kings could not read or write Latin, making it less important for citizens to learn.

Answer 1

Europe became more separated by languages, such as Spanish and French.

After the Roman Empire fell, Vulgar Latin diverged regionally into the Romance languages and local vernaculars became dominant. At the same time centralized schooling and Roman institutions collapsed, so Latin remained mainly a learned language of the Church and elites rather than the everyday tongue.