Just because you know a good sentence when you read one doesn't mean that you think it's easy to put one together - forget about writing an essay's worth. Learn how to write clear sentences and turn rough ones into gems.
How to Write Clear Sentences
A good essay, you can't begin to start writing one, not really, unless you learn to write sentences clearly.
That is a bad sentence. When you hear me speak it, or when you read it on the screen, you probably feel that instinctively, but how would you fix it? Let's take a second to analyze it.
The sentence is actually grammatically correct, but its priorities are out of whack. The first clause is 'a good essay,' which you assume is the subject - we're going to talk about a good essay. The next clause tells you that you can't begin to start writing one, but you don't know in what way the author means. Is it because you don't speak English? Because your best friend was killed by a rogue sentence? Because you refuse as a matter of religious principle? It's not clear.
Next, you have 'not really,' which is an aside - a bit of the author's voice that doesn't add anything to the sentence (it actually makes it more confusing) - and then, finally, 'unless you learn to write sentences clearly,' where the point of the sentence is finally revealed, but it ends on the adverb 'clearly,' which may be confusing ('does it modify 'sentences' or 'write'?' you might ask). The connection between the sentence's subject and its intent couldn't be further apart. Here's the same sentiment, cleaned up:
You can't write a good essay without learning how to write clear sentences first.
Or, more stylishly:
Clear sentences are the foundations of great essays.
Here's the thing: while we all wish that we could write with perfect clarity every time we sat down to a pen and paper, that's not realistic. Writing good, clear sentences is less about learning how to make sparkling, unicorn rainbows of text pour from your fingers than it is about learning how to fix and refine your writing after you've put it down (though you'll get better the more you practice and eventually turn out pretty good sentences on the first try more often than not). To put that more simply: clear sentences emerge from careful editing.
Clear Sentences Checklist
Here's a checklist of questions you can ask yourself to make your writing easier for your reader to understand.
Do all of the words in the sentence serve a purpose?
As we saw in the example sentence, one of the reasons it was confusing is there was additional information that didn't serve the intent of the sentence. When you're trying to fill space in a paper, suddenly adjectives, adverbs, and unnecessary asides often start popping up. Worse, many students, in the course of trying to reach minimum length requirements for a paper, start writing sentences with no substance at all. Let's look at an example:
In the course of the novels we are given multiple chances over and over to become suspicious of Snape's character, most importantly the multiple times that he harasses Harry and his friends for seemingly no reason at all even though they hadn't done anything to him.
The sentence, an analysis of A-Popular-Book-Series-That-Shall-Not-Be-Named, is pumped full of redundancies and ambiguous phrases that take away from the clarity of the sentence. First, we have 'multiple chances' and 'over and over.' As the writer, maybe what I think I'm saying is that Snape gives multiple reasons to doubt him on several separate occasions. Or maybe what I mean is that he does certain things enough times - i.e. 'over and over' - that there are multiple reasons to doubt him. It's not clear, so it has to go. Let's get rid of 'over and over.'
Next, we have 'most importantly the multiple times.' We've already used 'multiple' in the sentence once, so let's replace that with a synonym like 'many.' Also, what does 'most importantly' tell us? It seems to refer back to the 'multiple reasons,' but what is most important? If what the writer really means is to point out specific instances, he must be specific. Let's replace 'most importantly' with 'including.'