It’s actually very simple: there are some pronouns that we use after prepositions, and some that we don’t. In technical terms, prepositions have an object, and we use the dative form of pronouns to show this. Now, since I won’t be explaining either objects or dative forms for a while yet, this may not make a lot of sense. Luckily, there is an easy way to remember this:

After a prepositions, use the –m pronouns (me, him, them, whom) and the other ones like them: us, her, you, it (you and it don’t have a dative, or ‘m’ version). It does not matter how many pronouns you have.

In other words, after a preposition, use me, us, you, him, her, it, them or whom. Do not use I, we, he, she, they or who.

Usually, there is no problem with only one pronoun:

• The photographer caught a relaxed moment between them. (‘between’ is the preposition, connecting ‘the moment’ and ‘them’)
• The president shared a joke with her. (‘with’ is the preposition, connecting ‘the joke’ and ‘her’)
  

The confusion seems to come when there is more than one pronoun, and the second pronoun is separated by other words from the preposition, particularly when you is one of first pronouns, possibly because you does not have a different form after a preposition.

• The photographer caught a relaxed moment between her and him.  NOT between her and he.
• The photographer caught a relaxed moment between you and me. NOT between you and I.
• The photographer caught a relaxed moment between me and you.  NOT between I and you.

Note: as I discussed in a previous post, there is no grammar rule about the order of pronouns. It is simply a matter of manners or whom you want to emphasise.

Who and whom

While it should be simple to remember to use whom after a preposition, it is easy to become confused when the preposition is separated from the pronoun (its object) due to different word order: for example, in a question.

• You have not yet decided with whom you will share your pancakes. (sentence form)
• Have you decided yet with whom you will share your pancakes? (question form)
• With whom will you share your pancakes? (variation of the question)
• Who will you share your pancakes with? (standard spoken form of question: technically, not grammatically correct)

In spoken English, it is very common to begin a question with Who, rather than with a preposition, and to have the preposition at the end of the question. The problem is that we then forget that the preposition and pronoun are linked, and we generally use the grammatically incorrect form of who:

• Who will share your pancakes with?

The most common ungrammatical question is perhaps this one:

• Who are going with?

But, as I have said before, grammar is for written English, not so much for spoken English, and particularly not for a casual, informal, conversational question like Who are you going with? A previous edition of the Australian Government Style Manual was at pains to say that changing the question to correct the grammar – With whom are you going? – completely changed the tone, by making it more formal and quite ‘frosty’.

So: should you use Whom at the start of a question that finishes with a preposition? Well, between you and me, it completely depends on to whom you are talking and how you want to sound. (See what I did there?) Do you want to sound a little bit formal, even at the risk of sounding a bit frosty? Or do you wish to be casual and relaxed? This is the one situation where you can make up your own mind about what type of pronoun to use with the preposition: otherwise, always use the –m pronouns (and the related versions).

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Images: US Government image of President Obama and Michelle Obama, from rawpixel.com;  pancakes by Albany Capture on Unsplash