I have a lot I could say on this. Of most importance:
1) Style
Generally, in scientific writing, you want to just say things as they are, with great clarity. This requires clarity of thought on your part as to what you are writing about, and then you present the material in a way that is really easy to understand. Good scientific writing is really easy to read and understand - its purpose is to convey information.
Keep rereading what you've written and think, could you write it in a clearer way, or in fewer words (though sometimes more words can be clearer)? Write the essay, leave it for maybe a few days, come back and read it again - ambiguities in your writing may become apparent. Don't write anything for the sake of it, so delete anything redundant.
Your department might have guidelines on style. Follow these if they give them. Otherwise, I would write in third person, except in certain circumstances for special emphasis. However, sometimes a different style of essay may be wanted e.g. a more discursive essay comparing different options and coming up with your own judgement. That said, you can write your opinions in a detached, 3rd person manner, for more formal scientific writing.
Literature reviews
The point of a literature review is to systematically categorise the existing literature, such that it is clear where the gaps are e.g. "a, b & c did x, d, e & f did y, no one in the literature has yet done z" or "literature in this field broadly comes under either y or z". These gaps are what you suggest future research fills. Schematics/diagrams, or graphs comparing performance, are impressive. You can also get high marks by trying to find disagreement in the literature, and also agreement e.g. "a & b think x, disagreeing with c & d who think y". Better still is to try to explain why this disagreement might have arose.
Context
In my reports, I tried to appreciate the broader context of the topic/experiment, discuss possible applications, future work etc. However, when you are just starting learning a subject, this can be challenging.
2) The best written papers are easy to understand. If you find an article hard to understand, it may be the fault of the author for not presenting their ideas more clearly. Therefore, don't blame yourself/be disheartened!
I find reading all the way through an article, underlining what I don't understand, and moving on, helpful for a first reading, as opposed to getting stuck on a section I don't understand at first. I find that the article then tends to make much more sense upon rereading, now that I have the bigger picture. Rereading at a later date also helps. I look up the words I don't understand. Often, you don't need to understand the entirety of an article to write your essay. Often I find looking at citations of such papers helpful - they may explain something more clearly. Or looking at earlier work of the authors may be helpful.
Also, reading journal articles trying to understand them is not a waste of time - it can be a key part of writing a literature review, and increases your knowledge/understanding, but it is possible to become much more efficient at doing this.
If you really don't understand a particular article, just pick other articles to write about in detail, instead! You could just mention the incomprehensible article in passing, if at all.
3&4) I usually work the other way round - look at the literature, and use this to inform your plan. It's a bit different though if it's a literature review as part of a research project - I will then specifically look up all the key features of my research project in the literature, and see to what extent they have been adressed. Then I can say how my project will fill gaps. In which case, yes, I would be looking for articles to fill paragraphs on certain topics.
And for other kinds of essays, yes, once I've identified what the major things address are, I will look in the literature to help address them. But I use the literature to help me decide what to address in the first place.
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