Look at this example:
Tū es sapiēns. │ You are wise.
The adjective sapiēns is in the nominative case after the verb ‘to be’; this is called the predicative nominative.
When the sentence becomes indirect, the predicative nominative will also go into the accusative case:
Tū es ¦ sapiēns [nominative]. │ You are ¦ wise.
> Credō ¦ tē ¦ sapientem [accusative] ¦ esse. │ I believe ¦ that you are ¦ wise.
Further examples:
Magister meus ¦ homō doctus [nominative] ¦ est. │ My teacher is ¦ an educated man.
> Crēdō ¦ magistrum meum ¦ hominem doctum [accusative] ¦ esse. │ I believe ¦ that my teacher is ¦ an educated man.
Iūlia est bona discipula [nominative]. │ Julia is ¦ a good student.
> Magister dīcit ¦ Iūliam ¦ bonam discipulam [accusative] ¦ esse. │ The teacher says ¦ that Julia is ¦ a good student.
Tellūs ¦ magna ¦ est. │ The Earth is ¦ large.
> Legimus ¦ tellūrem magnam esse. │ We read ¦ that the Earth is ¦ large.
