Who do I write for? A reader I met recently called for less highbrow stuff, more basic grammar. Ok. But do I call him a reader I met/that I met/whom I met/who I met? Or maybe one I was talking to/that I was talking to/whom I was talking to/who I was talking to/to whom I was talking/to who I was talking? Which of those ways of saying it are correct English, which not?

It’s not just beginners in English who find that question difficult. It divides grammarians too. A pedant would say my very first sentence was wrong: it should read Whom do I write for? — nay, even For whom do I write? I disagree. In day-to-day English who can very often replace whom, be it in questions (Who did you meet?) or relative clauses (the reader who I met).

That’s not true, of course, the other way about: no way can you say Whom is there? or the reader whom met me. Nor does very often mean always. These days, you can indeed, with minor qualms, say a reader who I was talking to, but, not yet, a reader to who I was talking. That usage may be on its way, but to me, it’s still wrong: if you’re too pedantic to end your sentence with a preposition such as to, you must go the whole hog in pedantry and say to whom I was talking.

This apart, however, almost anything goes; every one of the phrases suggested in my opening paragraph is possible, except the last one. And, of course, if it’s a dog (or a wall) that you’ve been talking to, the word which avoids all uncertainties. People are who or whom (except in the curious traditional opening of the Christian prayer Our Father, which art in heaven...), but things, whether as subject or object, are simple which.

In turn, the word that can usually replace all three of the words. But it raises questions of its own. Clearly you can’t say the reader to that I was talking; you must say to whom. But can that really even replace who/whom/which in all other phrases? That’s not so clear.

There’s a broad rule. Suppose a postman comes to your door, and you find him very handsome. You want to tell your husband so (of course, you might not want to, but that’s a matter of life, not language). Then (1) you can say either the postman who came today is very handsome or that came today..., if you are using those phrases to define him: this postman, the one that came today, as distinct from 36 others. You have a choice; but in this case, that is preferable to who (or to which, if it was a dog you were admiring).

But if (2) you merely want to add some extra information about him, you’ll make pauses in your speech — in writing we mark them with commas — and say the postman, who came today, is very handsome. On top of declaring the postman (or the dog) — the usual one, no need to define him — to be handsome you’re just adding the fact that, by the way, he came today. And in this case who (or which) is not just preferable to that, but compulsory.

Simple? Alas, no. It’s a good rule, but not an absolute one. In the “defining” case, that is indeed preferable, but not always so. If the wording would otherwise include a lot of thats, use who: for example, she told me that the postman who’d come that morning was very handsome.

More surprising, some writers do use that, not who/whom/which, even in the “information-adding” case. In their influential book The King’s English, the Fowler brothers a century ago disapprovingly cited examples: De Quincey (her better angel, that hid his face with wings,...) and Thackeray (my stock of money, that card-parties had lessened by five-and-twenty shillings,...). The Fowlers’ disapproval was justified. But, however rare, the usage exists.

Or did exist. Happily, there are still broader rules to guide the inexpert here: (1) 19th-century usage may be 21st-century nonsense; (2) subject to that, when you can write as well as De Quincey or Thackeray (or, for that matter, say, Vikram Seth), feel free to imitate their idiosyncrasies; until then, don’t.