Learn Russian: Мы радуемся самой важной победе. - We are happy about the most important victory.

The verb радоваться (to be happy / to rejoice) is special. Instead of using a preposition like 'about', it directly requires the Dative case for the thing you are happy about.

How does the phrase самой важной form 'the most important' here?

In Russian, you form this type of superlative ('the most') by using the word самый followed by an adjective. Both must match the noun they describe. Because победе is a feminine noun in the Dative case, самый becomes самой and важный becomes важной.

Why does радуемся look so different from its infinitive радоваться?

Verbs that end in -овать have a special spelling rule. In the present tense, they drop the -ова- and replace it with -у- before adding the standard endings. So, радова- becomes раду-, then gets the 'we' ending -ем.

Why does the reflexive ending on радуемся use -ся instead of -сь?

The choice between -ся and -сь depends on the last letter of the conjugated verb. Because the 'we' ending -ем ends in a consonant (м), we attach -ся. If it ended in a vowel, we would use -сь.