Il n'y a personne qui puisse: nobody, questions, commas

Personne, rien, aucun: nothing to point at

When the antecedent has just been declared not to exist, there is nothing for the clause to state a fact about. Written French takes the subjunctive.

• Il n'y a personne qui puisse m'aider. - There is nobody who can help me.
• Il n'y a rien ici qui me plaise. - There is nothing here that I like.
• Je ne connais aucun livre qui dise cela. - I know of no book that says that.
• Il n'y a rien que je puisse faire. - There is nothing I can do. (que works like qui)

Speech often keeps the indicative: il n'y a personne qui peut m'aider is heard every day, but it is not the written standard. A weaker word such as peu de takes either mood, the subjunctive closer to hardly anyone: Il y a peu de gens qui savent le faire. or Il y a peu de gens qui sachent le faire. - Few people know how to do it.

Is there anyone who...? Questions

A question about whether something exists asserts nothing, so careful written French can use the subjunctive there too. It goes with the formal question; everyday questions keep the indicative.

All three mean Is there a train that leaves before noon?

• Connais-tu un médecin qui puisse venir ce soir ? - Do you know a doctor who could come this evening?
• Oui, il y a un train qui part à onze heures. - Yes, there is a train that leaves at eleven. (found: indicative)

Commas: the descriptive clause

A clause without commas narrows down which thing you mean, and can take either mood. A clause between commas adds a fact about a noun already identified, so it takes the indicative (or a conditional), never the subjunctive.

If the clause sits between commas, write the indicative and move on. The other fixed case, after le seul, le premier or a superlative (la seule chambre qui soit libre), has its own page.

Mistakes English speakers make

The opposite slip is just as common: once you know the rule, a subjunctive in every relative clause. The moment the thing exists, the indicative returns.