英語が必要な仕事なので、

[theseus_]

Context:
英語が必要な仕事なので、仕事中暇があれば英語の勉強をすることにしました。

My translation:
Because the job which requires English, I've decided to learn English when an idle moment turned up at work.

From the context, I inferred the meaning of "英語が必要な仕事なので" is "because the job which requires English".

But if "が" can mark the subject of a sentence, can the underlined clause also mean: "beacuse English is a necessary job" (just for example, it has no practical significance.)?

Moreover, if I change it into:

"英語が必要な仕事ですので"
"英語は必要な仕事なので"

Can them mean "beacuse English is a necessary job"?

 

[SoLaTiDoberman]

英語が必要な仕事なので、
=（今度の仕事は/私の仕事は/その仕事は）英語が必要な仕事なので、
＝Because the job requires English,
=Because English is a must for the job,

"英語は必要な仕事なので"
＝"英語は必要な技能なので"
=Because English is a must,
=Because generally speaking, English is a must,


I don't think they are interchangeable. The meanings becomes completely different.
You may think 今度の仕事は/私の仕事は/その仕事は is abbreviated in the original sentence, which can solve your question.

 

[gengo]

英語が必要な仕事なので、仕事中暇があれば英語の勉強をすることにしました。

SoLa has given you a good answer, but I'll just add that learning the difference between は and が is probably the final hurdle in becoming proficient in Japanese.

は always indicates what the speaker is going to talk about, and in the phrase SoLa gave (私の仕事は英語が必要な仕事なので), we see that 私の仕事 is the topic of the sentence, whereas the が refers to something about that topic. Literally, "As for my job, it is a job that requires English, so..." In such constructions it is common to omit the は clause because it can be easily understood.

To express your idea of "because English is a necessary job" (ignoring the fact that English isn't a job), we would say 英語は必要な仕事なので.

 

[theseus_]

I don't think they are interchangeable. The meanings becomes completely different.
You may think 今度の仕事は/私の仕事は/その仕事は is abbreviated in the original sentence, which can solve your question.

Thanks!
This is a very effective way for me to solve the question.

は always indicates what the speaker is going to talk about, and in the phrase SoLa gave (私の仕事は英語が必要な仕事なので), we see that 私の仕事 is the topic of the sentence, whereas the が refers to something about that topic. Literally, "As for my job, it is a job that requires English, so..." In such constructions it is common to omit the は clause because it can be easily understood.

To express your idea of "because English is a necessary job" (ignoring the fact that English isn't a job), we would say 英語は必要な仕事なので.

It is true that I always confuse with は and が.
Books say that は is the topic mark, and が is the subject mark, but the issue for me is how to understand them.
Thanks for the detailed explanation, it will help deepen my understanding of them.

 

[Flaminius]

Topic marker declares the marked topic as that for which all that is said thereafter is related. Practically, -wa divides a sentence like so:
英語は | 必要な仕事