Is there something unusual about the phrase: 'He is to blame'? It seems to be a shorter form of the passive 'He is to be blamed'. Does this make it some kind of adjective with a to-infinitive form? Are there any other phrases that follow this pattern?
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The idiom conveys the same meaning as the expression "to be blamed"
Look at this example from the same source-
Therefore, your sentence- "He is to blame" is just a concise way of essentially saying the same thing- "He is to be blamed".