になっているんですよ
ni natte iru n desu yo
JA
polite explanatory phrase meaning "it has become/it is in a state of" with a friendly emphasis
今日はすっかり春になっているんですよ。
It's already completely spring today, you know.
Etymology
It is a combination of the continuative form of the verb なる (to become), the progressive auxiliary いる, the explanatory nominalizer ん, the polite copula です, and the sentence-ending particle よ for emphasis.
Memory Aid
Break it down: になっている means "is in a state of having become", んです adds explanation, よ adds friendly emphasis.
Related Words
になるなっているんですよ