Breakdown of Этот поступок показывает, что он уверен в своих друзьях.
он
he / it (for a masculine noun) / it (for a neuter noun)
друг
friend
этот
this [f: эта, n: это]
в
in / to (direction)
Questions & Answers about Этот поступок показывает, что он уверен в своих друзьях.
The English translation says "sure of", but the Russian uses в. Why?
The short-form adjective уверен (sure/confident) requires the preposition в followed by the Prepositional case. While in English you are "sure of" something, in Russian you are literally "sure in" it. That is why we use в and put the following words (своих друзьях) into the Prepositional case.
Why use своих instead of его for "his friends"?
The word своих is a reflexive possessive pronoun that refers back to the subject of the clause, which is он (he). If you used его (уверен в его друзьях), it would mean he is sure of someone else's friends. Using своих makes it clear that he is confident in his own friends.
Why does друзьях have a soft sign (ь)? What is the dictionary form?
The nominative singular is друг (friend), but it has an irregular plural form: друзья. When you decline this irregular plural into the Prepositional case, the soft sign stays in the stem, resulting in the ending -ьях to give us друзьях.
Could I use действие instead of поступок for "action" here?