Learn Portuguese (Portugal): Acho que o guerreiro é vulnerável a dano mágico. - I think that the warrior is vulnerable to magic damage.

In Portuguese, many adjectives require a specific preposition to connect to the noun they refer to, a concept known as nominal regency. The adjective vulnerável requires the preposition a to introduce the thing one is vulnerable to.

Why is the preposition not contracted to ao dano mágico?

In this sentence, a is just the preposition on its own. We do not contract it with the masculine definite article o to make ao because dano mágico is being discussed in a general, uncountable sense. If we were referring to a specific, previously mentioned spell's damage, it would be vulnerável ao dano mágico.

Why does Acho que trigger the indicative é instead of the subjunctive?

In affirmative sentences, verbs expressing belief or certainty like achar and acreditar are followed by the indicative. You only trigger the subjunctive if the phrase is negative, meaning a sentence like I do not think the warrior is vulnerable would become Não acho que o guerreiro seja vulnerável.

Are there other adjectives that follow this exact same pattern with a?