The following guidelines should help you. If the word that means the owner does not end in s, add an 's to it. Students work long hours. A few items still need to be finished. Boys will be boys.
Radios are a wonderful invention.
D. Possessive pronouns do not need an apostrophe. This section on apostrophes is from the materials of Joan Schell Style Points If you lost style points, you will find that the number lost is equal to the sum of negative numbers in the right margin of your paper. If you are in ENL 111, at some point in the course, after we have worked on the syntax materials, you will be able to earn those points back. To get those points back, you must analyze the structure of the sentences in which you made the errors, i.e., the sentences with the negative numbers next to them. You must do this on the same piece of paper that I graded.