I'm currently studying to pass the TOEFL test with a high score, but I have encountered a sentence in an app that I fail to see why it is correct: "The committee members resented the president's not informing them of the meeting." Can someone explain to me how the mentioned sentence works?
The title of your question refers only to the "'s" in the sentence, so I am going to explain this.
This is a common genitive. So, the president's is the genitive (possessive) to the gerund construction not informing them. The question is "Whose not informing them?", and the answer is "the president's". The confusing part may be the word not, but just think of dropping it:
The committee members resented the president's informing them of the meeting.
The construction not informing them is a gerund, that is a construction derived from a verb (to inform) that is used as subject or object, as in:
Sailing is fun. (subject position)
I enjoy singing. (object position)
The original sentence (which has the gerund in object position) could be rephrased in a subclause (taking care of the tense then):
The commitee member resented that the president had not informed them of the meeting.