Yes,they are可否写成Yes,they're

\u4e3a\u4ec0\u4e48yes,they are\u4e2d\u7684they are\u4e0d\u80fd\u7f29\u5199

\u82f1\u6587\u4e2d\u7684\u6240\u6709\u7f29\u5199\u5728\u6b63\u5f0f\u7684\u573a\u5408\u6216\u662f\u6b63\u5f0f\u7684\u6587\u4ef6\u90fd\u662f\u4e0d\u53ef\u4ee5\u4f7f\u7528\u7684\u8b6c\u5982\u5982\u679c\u8981\u7528i can't do it .\u5c31\u4e00\u5b9a\u8981\u5199\u6210 i cannot do it . \u8fd8\u6709\u53e6\u5916\u4e00\u4e2a\u539f\u56e0\u5c31\u662f\u8fd9\u4e2a yes they are.\u5982\u679c\u662f\u7f29\u5199\u5c31\u662f yes they\u2018re \u4ece\u8bed\u8a00\u7684\u6ca1\u8d76\u4e0a\u8fd8\u6709\u9605\u8bfb\u65b9\u4fbf\u4e0a\u90fd\u8bf4\u4e0d\u4e0a\u597d\uff0c\u56e0\u4e3a\u540e\u9762\u6240\u5199\u7684are \u5f88\u5bb9\u897f\u522b\u5ffd\u7565\u548c\u8ba4\u9519 \u8bc6\u522b\u7a0b\u5ea6\u6bd4\u8f83\u4f4e\u3002

apple---apples they're---they are\u7701\u7565\u56de\u7b54\uff0c\u5fc5\u987b\u5206\u5f00\uff0c\u4e0d\u80fd\u7f29\u5199\u3002

这得看具体情况，
如果后面还有内容的话，可以缩写，
但如果只是作为问句的省略回答，最好别这么缩写。

当然，也没必要规定的这么严格。

呵呵。

不可以，否定的答案可以。
做句子可以，例如：They're best friends.

如果后面还有内容的话可以缩写，反之，不可以

还可以写成 YES,they R

扩展阅读：thank you very much ... amy are these yours ... what day is it today ... my shoes are green ... yes they do ... yes we do ... what would you like ... they are sheep ... they are beautiful ...