Translation:It is hard to write essays.
In my experience:
難しい (むずかしい) is "hard" in the sense that something is challenging or difficult. Maybe it's above your skill level, or it could be really complicated.
大変 (たいへん) is "hard" in the sense that something is a laborious task, requiring a lot of time or effort, and/or something so important you can't afford to mess it up. (It also has a lot of other uses, depending on the context.)
A math problem you can't figure out would be 難しい.
A semester-long project worth 50% of your final grade would be 大変.
(For added confusion, 大変 is sometimes used to emphasize 難しい. You may encounter the phrase 大変難しい (たいへんむずかしい), which translates a bit like "dreadfully difficult.")
"To write essays is hard" does not sound natural in English. In English, we usually say something like "Writing essays is hard."
What's the difference between 作文, 文章, and 論文? They all seem to translate as "essay".
It basically makes the verb into a noun. It's used here to make the verb into a subject.
It is like the -ing gerund in English. 書きます i write 書くの writing(the act of)
So when you want the topic to be an activity like "X-ing Y" the format is "Y を Xの-form は" Is that always the case? At first I thought the を particle should be a は.
を is used for direct objects. In this sentence, the essay (さくぶん) is being written directly, so を is needed. は wouldn't sound as good. Plus, さくぶん isn't really the topic here. The event of WRITING the essay is. I hope that helps.