With pronouns, that is.
A pronoun is a word that stands for a noun. Example: ‘The guy [noun] I was talking to is a third-year associate; he [pronoun] works in the corporate department’.
That and who
These pronouns have distinct uses: that refers to things and who to people.
So don’t write this: ‘Thanks to all that came to the event’. It’s ‘all who came to the event’ (unless they were robots).
An entity is an it
Not a they. Thus, ‘The bank was subject to a class action alleging that it had overcharged its customers for foreign-exchange transactions’.
You may have read that the ‘singular they’ is now a thing. It’s a thing, all right, but it’s sloppy and incorrect (unless you’re referring to a specific transperson who prefers to be called they; that’s cool).
They must otherwise always refer to two or more persons or things. Where there is only one, and the sex of the person is unknown, logic and grammar demand he or she – or else some gender-neutral but grammatically sound construction.
For example: ‘The company is seeking a new marketing director. Anyone interested in applying should submit his or her [OR JUST an] application by Friday and make himself or herself [OR JUST become, with no pronoun] familiar with the company and its competitors’.
This is what’s called a relative pronoun, which tells us more about the noun it relates to (‘a lawyer whose time has come’) or asks a question about it (‘Whose book is this?’).
Ideally, whose should be used only in reference to a person, not a thing; of which is the logical construction for the inanimate. So, ‘the Securities Act, the purpose of which is to regulate …’ rather than ‘the Securities Act, whose purpose is to regulate …’
That can lead, however, to artificial and clumsy sentences: Fowler’s example in Modern English Usage is ‘The civilians managed to retain their practice in Courts the jurisdiction of which was not based on the Common Law’, which he says could usefully (and perfectly correctly) be changed to ‘whose jurisdiction’ (I’d also take the capitals off ‘Courts’ and ‘Common Law’, which are wholly unnecessary).
So if you haven’t always been using of which for the inanimate, you can relax a bit – but don’t get too casual.
Next: get to the point