I recently joined an online group for writers and editors. As I was reading the group rules, I came across this sentence:
"Your post should allow discussion and debate among members without them needing to visit an external site."
You may think this reads perfectly fine. But it's not fine. Especially for a site by and for writers/editors.
The correct way to cast the sentence is like this:
"Your post should allow discussion and debate among members without their needing to visit an external site."
Sound weird? Too bad. It's correct. In this sentence, the word needing is being used as a noun, aka a gerund, rather than a verb. Therefore, the pronoun preceding it needs to be possessive, hence their instead of them.
Besides, if needing were being used as a verb, the subject would not be them; it would by they (and, of course, would require the helping verb are: they are needing). But obviously that messes up the whole sentence.
So, now that that's crystal-clear, I look forward to your never making this mistake.