The other day, I came across the sentence below:
I can understand you not telling him.
This is not correct. The sentence should read:
I can understand your not telling him.
Why? Because telling is a gerund, and when a pronoun precedes a gerund, the rule is to use the possessive case (your not you).
You may well ask, “What is a gerund?” In short, a gerund is a present participle used as a noun.
“Hold on there!” you say. “What is a present participle?” That’s another good question, and here’s the answer: A present participle is a verb stem with ing attached to the end of it–in this case, telling. Notice that with a present participle, the action is in progress right now.
Also notice that the participial phrase your not telling him is the object of the verb understand. In other words, it’s being used as a noun. That makes telling a gerund, and that in turn takes us full circle: the gerund telling calls for the possessive pronoun your.