I have seen this phrase and I wondered if it was correct: "Juro que él hubiera sido el primer fan" but to me, this was grammatically incorrect as the verb "jurar" does not require the subjunctive. Or am I wrong? I also thought this might be because natives prefer "hubiera" instead of "había", but again this is only a guess of mine and needs confirmation. Thanks.
"Juro que él hubiera sido el primer fan"
is correct if a condition is implied, and if "jurar" is used to express present certainty rather than certainty about a past event:
- Juro que él hubiera sido el primer fan si la hubiera conocido. (The speaker claims that, if he had met/known her, he would have been her number one fan, but he didn't.)
In a context in which the condition is implicit, the sentence proposed would be perfect to refer to a past hypothetical scenario:
1'. Es una pena que Bob no haya conocido a Whitney Houston. Juro que (de haberla conocido) hubiera sido el primer fan.
The indicative would be used to refer to a real past event:
- Juro que él había sido el primer fan. (I assure you he had been her first fan at the time.)
Note: To clarify the meaning of "jurar" as requested by @aparente001 in a comment, my understanding is that in sentence (1) the speaker expresses a present belief about a hypothetical event that never occurred: "Hubiera sido el primer fan, estoy seguro" (He would have been the first fan, I'm sure). Instead, in sentence (2) the verb "jurar" is used to ratify something that actually happened: "Te aseguro que había sido el primer fan" (I assure you he had been the first fan).