Questions & Answers about Я уверен в этом решении.
Why is "этом решении" in the Prepositional case?
The short-form adjective уверен (confident or sure) must be followed by the preposition в and the Prepositional case to express what you are sure about.
How do we know the gender of the speaker in this sentence?
The speaker is male because he uses the masculine short-form adjective уверен. A female speaker would add an "-а" and say я уверена.
Why does "решении" end in "-ии" instead of the standard Prepositional ending "-е"?
The noun решение (decision) is a neuter word ending in -ие. In Russian, nouns ending in -ия or -ие in the Nominative case change their ending to -ии in the Prepositional case.
Could I use the long-form adjective "уверенный" instead of "уверен" here?
Not in this sentence. You use the short form уверен to talk about a temporary state or feeling directed at something specific, like being sure of a decision. The long form уверенный is used to describe a permanent character trait, like a confident person (уверенный человек).