1. "S" is used to make a plural of the word student. 2. Here "s" indicates a possessive structure stating that the pen belongs to Peter. 3. "S" is used to make a plural of the word keyword. 4. "S" is added to the word study because the sentence is written in present simple tense and is about a third-person singular noun. 5. Indicates plural possessive action because the teacher answered questions that were asked by more than one student.