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Question

"Elle est la seule personne qui puisse m'aider = She is the only person who can help me"
 
Pourquoi pouvons-nous utiliser le subjonctif dans cette phrase ?

Answer

Evan,

the use of the subjunctive reflects the doubt that that she is in fact ‘ the only person ‘ who can help.

The first link below might help - see ‘ limiting ‘ adjectives section in particular. The 2nd link is focussed on superlatives but it is the same reasoning.

https://www.lawlessfrench.com/grammar/superlative-subjunctive/

Expressing "the most/least that I have (ever)..." with a superlative followed by the subjunctive mood in French (Le Subjonctif)