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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 ?

Accepted 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/
 
 https://progress.lawlessfrench.com/revision/grammar/expressing-the-most-least-that-i-have-ever-with-a-superlative-followed-by-the-subjunctive-mood-in-french-le-subjonctif