Under what condition should we use "that" as the required and ONLY relative pronoun? Please give some examples. Thanks!
There is NO circumstance in which the language requires a relative that and excludes use of a wh- relativizer or ‘null’ relativizer (no relativizer).
That is preferred by some writers (and most speakers) when the relative clause is restrictive, and this preference has been elevated to a ‘rule’ by some publishers and some writers on style; but the practice is not so generally observed that it can legitimately be called a ‘rule’ of the language itself.
There are contexts in which relative that is prohibited: that may not head a non-restrictive relative clause or follow a preposition at the head of relative clause. These are actual ‘rules’.
I would use "that" as the only acceptable relative pronoun in the following sentences:
- "Everything that I said was criticized."
- "All the figs that fall are eaten by the pigs."
- "This is the best hotel that I know".
Using "which" in these examples would be grammatical but would also sound awkward to my ears. ("which" just doesn't sound correct after certain words like "everything", "all", "little", "much" and after "superlatives") Certainly, "that" can be omitted in the first and third examples, and the sentences would sound better.