I encountered the following sentence recently: 这个重大责任， 就是对民族的责任。 I loosely translated it to: "This important responsibility is the nation's responsibility." But grammatically, I was confused by the use of 就是 and 对. I would have written it without those words. What grammatical function do 就是 and 对 play in the sentence?
"这个重大责任， [是]民族的责任。" - "This major responsibility, [is] the people's responsibility "
"这个重大责任， [就是]民族的责任。" - "This major responsibility, [exactly is] the people's responsibility "
"这个重大责任， [就是][对] 民族的责任。" - "This major responsibility, [exactly is] the responsibility [for] the people"
In all cases, '这个重大责任' is the topic of a [topic + comment/opinion] sentence
The comment/ opinion:
1b. '是民族的责任'" (is people's responsibility) is the comment on the topic
2b. '是' = 'is' ; '就是' = 'exactly is' ; '就' has many meanings. However, in this sentence, it means 'exactly', which emphasize the verb '是', not only "it is", it "exactly is"
3b. '对'(toward) = 'for' ; it is a preposition that indicates which object this responsibility is for
民族的责任 = the people's responsibility
[对]民族[的]责任 = responsibility [toward/ for] the people
Rephrase in English grammar: "This major responsibility, is the responsibility for the people"
就是 is more like "that is" in English. It's an emphasis on the explanation of the afforementioned "责任".
对...的责任 means "responsibility to ..."
就是 translates as "is" here. The character 就 may be what is confusing you, it's a very common character that is hard to explain but you will get a feel for it as you learn more Chinese.
This great responsibility is a responsibility to the nation (where nation implies "people" rather than "country“)
"Duty" would also be a valid choice rather than "responsibility", depending on the context.
Hope that helps.