For those who read the first Fifty Shades of Grey book (or watched the movie), and thought the ending was horrible, I’ll give you a spoiler that explains how there ever was a second book. (There was a second and then a third book, after all.)
If you’re ready for the spoiler, here you go: Ana didn’t use the safe words. Because she didn’t say anything, Christian kept going. Had she said something, he would have stopped.
Background: I listened to the first two books while driving around the country two or three years ago. I had been interested in something to listen to, and when I mentioned that, several women suggested that I try these books out. Unlike other people who thought the ending was horrible and never went on to the second book, I was curious about the ending to the first book, so I listened to the second one. Somewhere in the beginning of the second book Ana and Christian finally talk about what happened, and Christian says, “Why didn’t you use the safe words?”, at which point Ana realizes she is at least partially to blame for the ending of the first book. (Clearly they were coming together from two very different worlds.)
So, if you watched the first movie and thought the ending was horrible, and you have no plans to see the second movie, I hope that helps. And FWIW, the books are much better than the movie.