I need advice on how to understand articles written in the 1700s. I'm writing a history paper based on articles written in about 1790-ish. I'm having a hard time understanding the language, and comprehending what they are trying to say.
First, by finding and working with these articles you are off to an excellent start in the process of doing primary research. Although eighteenth-century English is "modern" English in both vocabulary and syntax, when you first encounter it, it can be difficult. One of the reasons is that this was a period when written English was still being standardized, and spelling, capitalization, and punctuation remained irregular until the early 19th century when some of the controversies over orthography were resolved.
The first thing to be said is that it gets far easier with practice. Rather than alternating between reading eighteenth-century and modern English, try immersing yourself in the period for a long weekend, so that 18th century English starts to sound natural to you. When you take breaks from your homework, perhaps watch videos of Restoration comedy or Jane Austen (original language, not modernized versions); after a few days, you will find it feeling far more natural.
For vocabulary, you will want to keep a full, unabridged dictionary nearby to look up archaic words. As well as an English dictionary, you will need a Latin one, as all educated English writers of the period knew the classics very well and often used Latin phrases and quotations in their writing.
Finally, one shift in English has been that people's syntactic attention span has been declining, and 21st century sentences tend to be shorter than the sentences of earlier centuries. As you read a long sentence, you need to concentrate on it, keeping all the elements of the sentence in your head until you reach the final period and have all the words necessary to assemble its meaning. The requires a sort of concentrated attention quite different from contemporary English, and can best be achieved by a distraction-free environment -- no music, phone off, no TV, etc.