のも当然だ means "it's only natural that", "no wonder": given what came before, the result is exactly what you'd expect. The clause in front ends in the plain form, past or non-past, and turns it into a noun; a noun or a な-adjective takes before の.
。
It's no wonder he's angry.
、。
You practised that much, so it's only natural you got better.
、。
He's lived in Japan for ten years, so no wonder his Japanese is good.
- × 日本語が上手のも当然だ → 日本語が上手なのも当然だ
The reason usually comes first, ending in , , or a conditional. It can also follow as a sentence of its own.
。。
It's no wonder he was tired. He was working from morning until night.
のも当然だ or のも無理はない
also means "no wonder", but it sympathises: the reaction is understandable. のも当然だ only says the result follows, and about someone's misfortune it can sound cold – they brought it on themselves.
、。
It's your first trip abroad, so it's understandable you're nervous.
、。
You didn't study at all, so of course you failed the exam.
Usage notes
These go beyond the plain form of the pattern.
The polite form is のも当然です. のも当然のことだ says the same at greater length.
、。
It's your first time, so it's only natural to make mistakes.
、。
You did everything yourself, so it's perfectly natural you're tired.
て当然だ
follows the て-form directly, with no の: "of course someone would". It also says what someone ought to do.
、。
After being told something like that, of course you'd be angry.
、。
You caused trouble, so you should naturally apologise.
Lingaku