Questions & Answers about Мальчик радуется самой новой игрушке.
Why is there no preposition for 'about' before the toy?
In English we say we are 'happy about' or 'rejoice at' something, but the Russian verb радоваться connects directly to the object that causes the joy. You don't need a preposition, you just put the object in the Dative case.
Why are the words самой новой игрушке in the Dative case?
The verb радоваться (to be glad/happy) requires the Dative case for the thing you are happy about. So, the noun игрушка becomes игрушке, and its adjectives must match it.
How does the superlative 'newest' work in this sentence?
What is the infinitive of радуется and why does the stem change?
The infinitive is радоваться. It belongs to a common group of verbs ending in -овать, which replace -ова- with -у- in the present tense (just like советовать becomes советую). Then you add the standard ending -ет and the reflexive particle -ся.