て当然だ says a result is only to be expected given the circumstances: "it's natural that", "no wonder", "of course". It goes on the て-form: an い-adjective becomes 〜くて, a な-adjective or a noun takes で.
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You practiced that much, so it's only natural that you got better.
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You haven't eaten anything, so of course you're hungry.
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It's your first time, so it's natural to feel anxious.
The first half usually gives the reason, with , or a conditional such as 〜ば or 〜たら.
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If it's this hot, it's no wonder you're thirsty.
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If it's this cheap, it's no wonder it's popular.
"Deserves it"
With a passive or a verb of receiving, て当然だ judges that someone has earned what they get, good or bad.
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If you break a promise, you deserve to be told off.
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If you make an effort, you deserve to be rewarded.
て当然だ, のは当然だ and て当たり前だ
takes a plain-form clause and turns the whole of it into the thing that is natural. It reads more like a comment from outside the situation.
。
It's natural that he's angry.
means the same as て当然だ and sounds more conversational.
、。
They're professionals, so of course they're good.
Lingaku