Grammar JLPT Level N3 - Lesson 14

Structure of だって:

Noun + （なん） + だって
だって + Phrase

Details about だって:

だって is a colloquial combination of the auxiliary verb だ, and the adverbial particle とて in Japanese (a combination of the particles と, and て). It has a wide range of uses, but primarily exclaims that something is the 'premise' for whatever has been said, or will be said next. This 'exclamation' is usually translated as 'I heard', 'cause', 'but', or 'even if'. The meaning that will be conveyed is determined primarily by where だって appears in a sentence.
When placed at the beginning of a sentence, だって highlights the speaker's (usually) displeasure about something that has just been said, before following it with an explanation or counterpoint. This だって is quite similar to でも.

え〜行(い)きたくないよ。だって、俺(おれ)の元(もと)カノも誘(さそ)ったんでしょう？
Awwww maaaan I don't want to go. It's because you also invited my ex-girlfriend, right?
だって、サメとか怖(こわ)いもん。
It's because I'm afraid of things like sharks.

At the end of a sentence, だって is used to express hearsay, in a similar way to 'I heard that', or 'it was said that', in English. In these cases it will be preceded by なん. なんだって always appears after a noun, or a phrase that has been nominalized (with の or こと).

レムの実家(じっか)は農家(のうか)なんだって。
I heard that Rems' family owns a farm.
キリコは明日(あした)から一週間(いっしゅうかん)休(やす)みなんだって。
I hear that Kiriko will be on break for a week starting tomorrow.

だって

Usage Notes for だって:

When used directly after a noun, だって has a similar nuance to the だって that appears at the beginning of sentences (emphasizing what has come before it, in order to explain further). This comes across as でも, and may not be used with なん.