Copy pasting definitions and theorems from other papers to my own paper I am writing a mathematical paper.  Since my English is not perfect, i am borrowing sentences from English-speaking authors. I have some questions about copy and paste from other papers to my paper. As you know, one needs some definitions and theorems from other papers. I cited all of papers that i used either definition or theorems from them.

1)Can i copy and paste the original either theorems or definition from the papers to my paper or i have to change them?i mean writing somehow that is not as same as original one.  Because i have seen English-speaking authors often change original theorem or definition in their papers. On the other hand, i have seen someone does not.

2)As i mentioned, i am borrowing sentences from English-speaking authors. When i look it my introduction, i see this sentence from that paper, that sentence from that paper and ... It makes scene. My supervisor read it and he said it is okay( it is not ambiguous) but i'm scared that referee rejects my paper.
Please do not borrow sentences

Instead, borrow sentence structures. If the original sentence is, "Our results also have implications on the study of öhkömönkiäiset.", you might write "Our main result also has implications on öhkömönkiäis-studies."

First, this way you are not copy-pasting. Second, this forces you to think about the language and therefore learn it, thereby reducing the need to copy and making writing a lot easier.

Also, go ahead and improve your English. There are many free resources online, but simply reading a lot is a great help. For more specific advice on how to learn the language, https://languagelearning.stackexchange.com/ is a fine place to ask. For specific questions about the English language, maybe ask at https://ell.stackexchange.com/.

(The easiest improvement to the English in this question would be to capitalize "I".)

Quoting definitions and theorems word-for-word

For many basic definitions, there are only a small number of ways of writing them clearly and compactly. Just use an established way of writing.

Sometimes, if a definition or result is complex or not yet established, there are better and worse ways of writing it and maybe even several variations that define slightly different things. In this case, cite the source, since the exact definition matters.

Usually you are somewhere between these two cases, in which case I would recommend you to write the definition or theorem by yourself, so that you are more likely to write it in the same way as your other results and to use the same notation.