Contracting 〜ているのだ to 〜てんだ

Grammar: Colloquial & Casual Speech

A verb ending in 〜て＋いる followed by the particle のだ may contract as てんだ.

First, the い in 〜て＋いる is dropped. Next, る becomes ん due to の following it. Finally, の becomes ん when followed by だ to です.