إِنَّ الطَّالِبَ نَاجِحٌ
Indeed, the student is successful.
Inna introduces a nominal clause, makes its noun accusative and reinforces the predication. Anna also belongs to the inna-type particles but commonly appears in a clause embedded within a larger construction:
عَلِمْتُ أَنَّ الطَّالِبَ نَاجِحٌ
I learned that the student is successful.
The distinction is not merely their written hamzah. Their syntactic positions also differ: a clause with anna can function as a verbal noun construction, while inna often introduces an independent assertion.