Hopefully I can explain this alright.
Take the following sentences, one in English and it's German translation.
In the 1800s, one of the early challenges was to explore the land and find suitable areas where people could settle.
Im 19. Jahrhundert war es eine der ersten Herausforderungen, das Land zu erkunden und geeignete Areale zu finden auf denen sich Leute niederlassen konnten.
I'm confused with why 'es' was used in the German. Perhaps I am over-analyzing this too much, but what exactly is the subject in the english version? I believe it to be the whole phrase 'One of the early challenges' or perhaps just 'One'. And in the German version 'Es' is the subject, correct?
But then that would make the german sentence's english equivalent seem moreso to me like this:
In the 19th century, it was one of the first challenges: to explore the land and to find suitable areas on which they the people could settle.
But are these 2 english sentences (the first one given and this new version) consisting of the same subject? No, right? You have 'it' as the subject in the second version. And it can easily be seen how 'es' would then be in the translation acting as the subject. My point being that I'm unsure on how to translate particular english sentences (like the first english version) into german because I'm unsure of how to present the subject in german.
My question I guess is how do you know when you need to use 'es' as the subject in german?
What grammatical rule was preventing me from omitting 'es' entirely in the german version?