That Which Who Whom

Relative pronouns in English: a practical guide for Portuguese speakers

That, which, who, whom — four words that cause confusion for a lot of people learning English. The problem is that Portuguese handles relative clauses very differently, so the mapping isn't clean. You end up using the wrong one in situations where it sounds off, even if you can't explain exactly why. I'm going to walk through how each one works, when to use them, and where people consistently trip up. This isn't theoretical grammar. This is based on correcting student writing for years, where the same mistakes showed up again and again.

When to use that which who whom

The first thing to understand is that "that" and "which" are for things, and "who" and "whom" are for people. That's the basic split. But the difference between that and which, and between who and whom, is where it gets fuzzy for Portuguese speakers. That is used in restrictive clauses — clauses that are essential to the meaning of the sentence. Think of it as defining which specific thing or person you're talking about. For example: "The book that I borrowed is overdue." You're specifying which book. Without that clause, the sentence loses its point. Which is used in non-restrictive clauses, which add extra information that isn't essential. These are set off by commas. For example: "The book, which I borrowed from Sarah, is overdue." Here, "the book" is already specific. The clause just gives you extra detail.

Here's the thing most guides don't emphasize enough: American English tends to prefer "that" for restrictive clauses where British English would accept "which." If you're writing for an American audience, sticking with "that" for restrictive clauses is the safer choice. Mixing them up in published work draws attention from editors. Who replaces "that" when you're talking about people in restrictive clauses. "The person who called yesterday left a message." It's that simple. No comma before who in a restrictive clause. If you put a comma there, you've switched to non-restrictive mode, and the meaning changes.

The who versus whom distinction

Whom is the object form of who. In formal writing, it still has a place. In spoken English and most casual writing, it's been dropping out of use for decades. That said, certain contexts still expect it, and using the wrong form in those situations makes you look careless. Here's the test: if you can replace the pronoun with "him" or "her," use whom. If you can replace it with "he" or "she," use who. "The person who/whom I hired was late." Replace with "I hired him" — whom is correct. "The person who/whom called was late." Replace with "he called" — who is correct. The awkward reality is that this test breaks down in embedded clauses or when the pronoun appears at the start of a sentence with a preposition floating somewhere else. I spent an afternoon rewriting a legal brief where the attorney had inserted "whom" in a spot that was grammatically correct but reading it made the sentence stumble. The fix was restructuring the whole sentence, not just swapping the pronoun.

Common mistakes Portuguese speakers make

The biggest issue is that Portuguese uses "que" for almost everything English splits into that, which, and who. When a Portuguese speaker translates directly, they'll often write "the car which is red" when they mean "the car that is red," or they'll drop the relative pronoun entirely because that's acceptable in Portuguese but not in English. Another trap: Portuguese allows omitting the relative pronoun when it's the object ("o livro que comprei" can become "o livro comprei" in informal speech), but English requires it. "The book I borrowed" is fine because "I borrowed" clearly marks it as a reduced relative clause, but "The book which I borrowed" is also correct and sometimes preferred in formal writing. Preposition placement is another headache. Portuguese puts the preposition before the relative pronoun naturally ("a pessoa com quem falei"), but English speakers will say "the person I spoke with" rather than "the person with whom I spoke" in most contexts. The formal version isn't wrong — it's just stiff and can sound affected depending on the register.

Edge case that trips people up

There's a situation involving collective nouns where even experienced writers second-guess themselves. Take a sentence like: "The team that won the championship were celebrated in the city." Should it be "was" or "were"? In British English, collective nouns often take plural verbs when the emphasis is on the individuals. In American English, "was" is standard. Neither is strictly wrong, but mixing the conventions in a single document looks sloppy. I ran into this when editing a multinational company's internal communications. The US office wrote with collective nouns as singular, and the UK office used plural. The fix wasn't a grammar rule — it was establishing a style guide that picked one convention and stuck with it, with a footnote explaining the variation for anyone who questioned it.

A quick reference

Use "that" for restrictive clauses about things. Use "which" for non-restrictive clauses about things. Use "who" for people as the subject. Use "whom" for people as the object, particularly in formal writing. Omitting the pronoun works in restrictive clauses when it functions as the object, especially in American English. The biggest mistake I see isn't picking the wrong pronoun. It's not recognizing whether the clause is restrictive or non-restrictive, which determines everything else. Once you can identify that distinction, the rest follows naturally.