Japanese with Manga
Learning through examples in manga

Contracting 〜ているのだ to 〜てんだ

Grammar: Colloquial and 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.

Examples

Why are you silent with 〜てんだ

Manga panel from 三ツ星カラーズ showing example of Contracting 〜ているのだ to 〜てんだ.
三ツ星カラーズ » Volume 6 » Page 23

Being the first to arrive at the clubhouse after school, Yui decides to change her hairstyle and see how long it takes the others to notice when they arrive.

Sacchan arrives next and starts up a conversation. Yui simply sits there without saying anything.

さっちゃん:
「なんで黙(だま)ってんだ！」
“Why are you being silent!”

You've been found out with 〜てんだ

Manga panel from 名探偵コナン showing example of Contracting 〜ているのだ to 〜てんだ.
名探偵コナン » Volume 1 » Page 8

Shin’ichi reveals that the murderer escaped out the window and onto the roof, naming the wheelchair-bound husband as the culprit. The husband points his cast-covered leg, but Shin-ichi knows better.

新一(しんいち):
「もう、ネタはバレてんだ！！」
“Your deceit has already been found out!”
メイド:
「だ、だんな様(さま)、足(あし)が…」
“Master, your leg...”