Spoilには「台無しにする」という意味でAlertは「警告」です。
Spoiler Alert! は「ネタバレ注意！」になります。
「これはネタバレを含みます」は
This includes spoiler. になります。
また、Spoilerは秘密をばらしてしまう人を指すこともあります。
例: Don't tell John about Lucy's surprise party. He's a spoiler. He will tell Lucy right away.
ジョンにルーシーのサプライズパーティーのことを言っちゃダメだよ。彼はすぐにルーシーにバラしちゃうから。
とにかくSPOILERが入っていればOKです、なるべく見やすいところでね。
例：
SPOILER ALERT
In "The Force Awakens," Chewbacca is Kylo Ren's father.
…なんてね
Spoiler は英語の spoil という動詞（台無しにする）からの言葉です。
He spoiled the movie for me!
（彼のせいで映画が台無しに！）
名詞、動詞としても活躍する言葉です。
This article includes details about the plot, so please do not read it if you think this may lessen your enjoyment
This statement is really a disclaimer so that nobody will get cross with you or blame you for telling them important aspects about the plot- or whatever it may be. You are forewarning people of the content of your article, so if they read it, then that is their choice.
"I have to warn you that this article includes details about the plot, so please do not read it if you think this may lessen your enjoyment."
"OK, I think I'll read it after I see the film."
The article contains spoilers of a movie etc......
Warning, the article contains spoilers!
If you want to let someone know that the article contains spoilers then you can simply say
'Spoiler alert' this tells the person that there are spoilers in it. You can also say Warning, the article contains spoilers! by saying 'warning' you are telling them to be careful or to watch out,
You could also say 'The article contains spoliers of a movie etc......'
”Spoiler alert"（ネタバレ注意）と言えば十分伝わります。また、”Warning"（忠告）という言い方もあります。
”The article contains spoilers of a movie, etc...”という言い方も出来ます。
（この記事には映画のネタバレが含まれています。）
These are quite casual warnings, but they are pretty direct and clear.
If you wanted to say something that was more professional, you might try something like
''This article contains within it details of the plot, so you may wish to delay reading it until you have watched the film / read the book''
カジュアルな表現ですが、結構ストレートで明確な忠告です。
もっとプロフェッショナルな感じで言いたい場合―
”This article contains within it details of the plot, so you may wish to delay reading it until you have watched the film/read the book."（この記事は物語の詳細が含まれているため、映画をご覧になるまで／本をお読みになるまで、読まない方が良いかもしれません。）
と、表現することも出来ます。
Be warned. Reading further might spoil certain parts of the movie for you.
You probably should stop reading here if you don't like spoilers.
When you want to write at the top of your article that the piece will most likely spoil a movie for someone; then you may express it in the following ways:
-Spoiler Alert!
-Be warned. Reading further might spoil certain parts of the movie for you.
-You probably should stop reading here if you don't like spoilers.
記事の冒頭で、内容にネタバレが含まれていることを伝えたいなら、次のフレーズが使えます。
-Spoiler Alert!（ネタバレ注意）
-Be warned. Reading further might spoil certain parts of the movie for you.（注意。ここから先、映画のネタバレを含みます）
-You probably should stop reading here if you don't like spoilers.（ここから先、ネタバレを含みます）
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