qui que ce soit

1. Usage in Negative Sentences
Unlike personne (nobody), qui que ce soit is not a negative word itself. When used to mean 'nobody' or 'not anyone', it is typically paired with a full negation like ne... pas, other negative adverbs like ne... jamais or ne... plus, or a negative preposition like sans. Because qui que ce soit is itself a negative-polarity expression, ne alone is also correct with any verb and sounds more literary: Je n'y ai trouvé qui que ce soit. There is a key nuance difference: Je ne veux voir personne is the standard, neutral way to say 'I don't want to see anyone,' whereas Je ne veux pas voir qui que ce soit is a heavier, emphatic choice ('I don't want to see anyone at all'). Furthermore, Je ne veux pas voir n'importe qui means 'I don't want to see just anyone' (you are selective).
Example: Je n'en ai pas parlé à qui que ce soit. (I haven't spoken about it to anyone at all.) The same sentence without pas — Je n'en ai parlé à qui que ce soit — is equally correct and a touch more formal.

2. Usage in Hypothetical, Interrogative, or Doubtful Contexts
It can be used in 'if' clauses, questions, after verbs of doubt (e.g., Je doute que qui que ce soit vienne), or after conjunctions like avant que (e.g., avant que qui que ce soit ne le remarque) to refer to a non-specific person. However, this can sound heavy. In everyday speech, native speakers typically use quelqu'un unless they want to strongly emphasize 'anyone at all'.
Example: Si qui que ce soit appelle, dis que je suis occupé. (If anyone at all calls, say I am busy.)

3. Usage in Comparisons
Qui que ce soit is frequently used in affirmative comparisons to mean 'anyone'.
Example: Il le sait mieux que qui que ce soit. (He knows it better than anyone.)

4. The Subjunctive Requirement and Tense
When qui que ce soit is the antecedent of a relative pronoun (qui or que), the following verb must be in the subjunctive because the identity of the person is uncertain.

• Qui que ce soit qui vienne... (Whoever [it is who] comes...)
• Qui que ce soit que vous choisissiez... (Whoever [it is that] you choose...)

In formal or historical texts, you may encounter the literary past form qui que ce fût (using the imparfait du subjonctif).

5. Standalone Usage and Register
This expression is slightly elevated but common in standard speech (only the past form qui que ce fût is strictly literary). It can also be used as a standalone parenthetical phrase meaning 'whoever it may be' or 'no matter who'.
Example: Qui que ce soit, je ne veux pas lui parler. (Whoever it is, I don't want to speak to them.)

6. Subject Restriction and Related Terms
Qui que ce soit cannot be used as a standalone subject of a standard affirmative or negative sentence (though it can act as a subject when it is the antecedent of a relative clause, e.g., Qui que ce soit qui ait fait ça...). For 'Anyone can do it', you must use n'importe qui. For negative sentences, Personne must be used as the subject (e.g., Personne n'est venu). Additionally, note its counterparts: quoi que ce soit is used for things (whatever/anything), and quiconque is a one-word formal equivalent for 'whoever'. Crucially, quiconque can be used directly as a subject (e.g., Quiconque appelle...) and takes the indicative mood (for general, factual statements), whereas qui que ce soit cannot be a direct subject without adding qui and requires the subjunctive (implying doubt).