Here are some tips on how to offer constructive criticism. This is great information not only for you as a writer, but also for the friends and family you have reading YOUR book. That way they can support you as best they can.
First, keep in mind that, no matter how strongly you feel about your point of view, it is still only ONE point of view out of many. Unless you are pointing out a clear typo, just about anything else is up for grabs. Many writers twist grammar for playful reasons. Something you might feel is a historical anachronism might actually be a well researched gem. So in phrasing any and all suggestions, always strive for gentle kindness. Say something like "I'm not sure that tomatoes were known in 1200s medieval England ... I thought they were brought over centuries later from South America? Maybe you could use raspberries instead?" That way it encourages the author to look more into that issue. You also provide an alternative that could be more accurate.
You might be very upset that the character Jane ran off with Mark. Certainly it is valid to express that! The author wants to know how you relate to the characters. So you could say "It bothered me a lot when Jane ran off with Mark; it seemed to go against her honorable nature up until that point in the story. I would think she would resist the temptation. If you did want to show she had reached her limit, maybe add in more scenes to show that change in her character?" Again your point here is to help show your point of view. You connected with the character and felt you understood her. Then something happened which didn't make sense. You're hoping to help make that transition more smooth.
We all have dialogue preferences. For example, some of us favor heavily "accented" dialogue to represent the speech in a given area of the world. Others of us prefer to read the words simply as words and to "hear" the accent in our head. You can certainly present the style that you favor, but this is an area where the author tends to write in their own style. Unless you find the style makes the book completely unreadable for you because all of the 'tis and thee and thou, it can be best to let the author write in dialogue that makes sense to them. Unless of course they are using all the words in a completely ungrammatical way :). You might want to guide them in that case.
The author might get defensive at even your most carefully worded suggestions. That is only natural for a person who has created a world :). The author cares about her world deeply and believes in it. It can be challenging to hear that something might be "wrong" with her world. So try to phrase each suggestion so that it is aiming to polish the world to make it even better.
For example, you could say you understand it's not that it's "wrong" for Juliet to be a prostitute. Prostitutes were around since the earliest civilizations and they were tolerated as a normal part of society. In most early societies they were seen to provide a normal service as much as a butcher or baker provided a normal service. In many cases prostitution was the only way for a woman to survive and have food to eat. But maybe you feel that, in a strongly religious household, a character who was an open prostitute might be steered by her family into more Godly ways of earning her keep. If it seems like a disconnect exists in a relationship, the author can either tweak how the characters react to each other or add in more explanation so it's more clear why the relationship is the way it is. Maybe the issue is that there is a cultural norm for the book's setting that isn't being clearly enough conveyed.
Again, to reinforce the main point, as a reviewer it can be tempting to think that your view is the "right" view. Keep in mind that often ten people can read the same book and have ten different opinions on it. So certainly present your view on it! Your view is important, because undoubtedly a portion of readers will share your view. It's important to take your view into consideration. At the same time, it could be that 90% of readers have a different opinion, and that is fine too. So while your points should be considered, they won't necessarily be adopted. It's important to be OK with that as well. You are providing one view of many. You are part of the overall mix of ideas for polishing the book.
The more reviews you perform, and the more you hone your skills as a supportive, constructive offerer of ideas and suggestions, the more it will help your own writings sing and come to life!
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