"连接主句和从句的" can be translated into English as "that connects the main clause and the subordinate clause" or more concisely as "linking the main clause and the subordinate clause" (when used as a participial phrase to describe a function).
For example:
The conjunction that connects the main clause and the subordinate clause is crucial for understanding the sentence structure. （连接主句和从句的连词对于理解句子结构至关重要。）
In this sentence, the relative pronoun serves as linking the main clause and the subordinate clause. （在这个句子中，关系代词起到了连接主句和从句的作用。）
If you're referring to a specific grammatical element (like a conjunction or relative pronoun) that performs this function, you could also say:
The conjunction (e.g., "because," "although") links the main clause and the subordinate clause. （连词（如“because”“although”）连接主句和从句。）
The relative pronoun (e.g., "who," "which") functions to connect the main clause and the subordinate clause. （关系代词（如“who”“which”）起到连接主句和从句的作用。）