"Which" is used for things, animals, and concepts; "who" is used for people. The most fundamental relative pronoun rule in English.
Relative pronoun: introduces a relative clause modifying a NON-HUMAN noun (thing, animal, concept, place, event). "The book which I recommended" (book = thing). "The company which hired her" (company = organization, not a person). "The idea which she proposed" (idea = concept). In informal American English, "that" can substitute "which" in restrictive clauses.
The report which the team submitted last Friday contained several inaccuracies that need to be corrected.
This is the software which our IT department uses to manage internal requests.
The conference, which took place in Madrid, attracted more than two thousand participants from forty countries.
Relative pronoun: introduces a relative clause modifying a PERSON. "The person who called" (person = human). "The manager who signed the contract" (manager = human). "Everyone who attended" (everyone = people). Also works for fictional characters, deities, and pet animals when treated with personal character.
The colleague who presented the analysis this morning has worked in the industry for over fifteen years.
Do you know who approved the final budget? I need to follow up with them directly.
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Common mistake
Classic error: "The person which called" = INCORRECT. People always use WHO, never WHICH. "The book who I read" = INCORRECT. Things always use WHICH (or THAT), never WHO. The rule is simple: HUMAN = WHO. NON-HUMAN = WHICH (or THAT). Pet animals: "The dog who saved the child" is acceptable when the animal is given personal character, but "The dog which runs fastest" is equally correct. For animals in scientific or impersonal contexts, use WHICH/THAT.
Trick to remember it
Question: can what you are describing TALK? If it can talk (it is a person) = WHO. If it cannot talk (thing, object, concept) = WHICH. "The teacher ___ explained it" = the teacher can talk = WHO. "The explanation ___ helped me" = the explanation cannot talk = WHICH. Second trick: substitute with "he/she" or "it/they". If the natural substitution is "he/she" = WHO. If it is "it/they" = WHICH. "The person ___ helped" = she helped = WHO. "The tool ___ works" = it works = WHICH.
Quick rule
Who = people. Which = things, animals (in impersonal context), concepts. That can replace which or who in restrictive clauses (no commas).
Did you know?
In Old and Middle English, the who/which distinction was not as rigid: "which" was used for people in many contexts. "Our Father which art in Heaven" in the King James Bible (1611) uses "which" for God (a person). The modern distinction who = person / which = thing was progressively standardized between the 17th and 19th centuries, driven by the first prescriptive grammar books. Today, "which" for people sounds archaic or incorrect in modern English, except in liturgical or literary contexts.
Exercise 1 · Pick the right one
She is the project manager _____ led the team through the most difficult phase of the expansion.
Exercise 2 · Pick the right one
The training program _____ she completed last year gave her a significant competitive advantage.
Exercise 3 · Form the word
Who = people. Which = things, animals (in impersonal context), concepts. That can replace which or who in restrictive clauses (no commas).
Classic error: "The person which called" = INCORRECT. People always use WHO, never WHICH. "The book who I read" = INCORRECT. Things always use WHICH (or THAT), never WHO. The rule is simple: HUMAN = WHO. NON-HUMAN = WHICH (or THAT). Pet animals: "The dog who saved the child" is acceptable when the animal is given personal character, but "The dog which runs fastest" is equally correct. For animals in scientific or impersonal contexts, use WHICH/THAT.
Question: can what you are describing TALK? If it can talk (it is a person) = WHO. If it cannot talk (thing, object, concept) = WHICH. "The teacher ___ explained it" = the teacher can talk = WHO. "The explanation ___ helped me" = the explanation cannot talk = WHICH. Second trick: substitute with "he/she" or "it/they". If the natural substitution is "he/she" = WHO. If it is "it/they" = WHICH. "The person ___ helped" = she helped = WHO. "The tool ___ works" = it works = WHICH.
Bryan López
Founder of SpeakUP Academy