这种专业文章如果能提供一下出处（http://seminariauh912.wikia.com/wiki/M._Ahaniyan） 会更方便其他人理解其上下文。

仔细从原文中寻找了一下大写的 Monitor 的定义，发现它指的是“对自己输出的语言进行有意识地语法修正的行为”。

提问的语句的完整语境是：

However, there is a place for grammar, or the conscious learning of the rules of a language. Its major role is in the use of the Monitor, which allows Monitor users to produce more correct output when they are given the right conditions to actually use their Monitor, as in some planned speech and writing. However, for correct Monitor use the users must know the rules they are applying, and these would need to be rules that are easy to remember and apply--a very small subset of all of the grammatical rules of a language. It is not worthwhile for language acquisition to teach difficult rules which are hard to learn, harder to remember, and sometimes almost impossible to correctly apply.

由上下文可知，its role 指的是“语法的角色”，整句是说：

“语法的角色主要用于语言监控，可以令使用者能在合适的条件[Conditions(=situations)]下能对自己的语言实施监控[并修正 （edit）]，从而输出更加正确的语言。所谓“合适的条件”，举例来说，比如（as)写提前计划的演讲词或其他文字的时候。

参考原文介绍的“监控假说”的定义（注意标粗的部分，可以很好地帮助理解上面的引文）：

'3.     'The Monitor Hypothesis:

The language that one has subconsciously acquired "initiates our utterances in a second language and is responsible for our fluency," whereas the language that we have consciously learned acts as an editor in situations where the learner has enough time to edit, is focused on form, and knows the rule, such as on a grammar test in a language classroom or when carefully writing a composition. This conscious editor is called the Monitor. Different individuals use their monitors in different ways, with different degrees of success. Monitor Over-users try to always use their Monitor, and end up "so concerned with correctness that they cannot speak with any real fluency." Monitor Under-users either have not consciously learned or choose to not use their conscious knowledge of the language.

Although error correction by others has little influence on them, they can often correct themselves based on a "feel" for correctness. Teachers should aim to produce Optimal Monitor users, who "use the Monitor when it is appropriate and when it does not interfere with communication." They do not use their conscious knowledge of grammar in normal conversation, but will use it in writing and planned speech. "Optimal Monitor users can therefore use their learned competence as a supplement to their acquired competence."