"must have been" construction

Question

Passage in question:

よっぽど　おなかが　すいて　いたんだなあ。

As I understand, the translation is something along the lines of "You must have been very hungry." My problem lies within the bold text: how exactly each element of this passage is constructed? Dictionaries and rikaikun didn't quite help and I also don't know whether there are any kanji in the passage (although I doubt it) since I encountered it in a children's cartoon with kana-only subtitles.

Answer 1

• 空【す】く: (consonant-stem, intransitive, "instant state-change" verb) "to become empty/vacant".
• お腹【おなか】が空く【すく】 : (verbal set expression) "to become hungry". See: Why is it お[腹]{なか}が[空]{す}いた but [腹]{はら}[減]{へ}った?
• お腹が空いて: Te-form of お腹が空く.
• お腹が空いている: "to be hungry". See When is Vている the continuation of action and when is it the continuation of state? and Difference between お腹がすいた and お腹がすいている ― 「お腹がすいた」と「お腹がすいている」の違いは？
• お腹が空いていた: Ta-form (past form) of お腹が空いている. "to have been hungry".
• んだ: =のだ. "It is that ～." This is known as explanatory の. See: What is the meaning of ～んです/～のだ/etc?
• なあ: Just another sentence-final particle to show exclamation.

So the super-literal translation of お腹が空いていたんだなあ would be something like "Oh, it is that your stomach had become empty."