Ideas
Let's start with an assertion:
Sentences express ideas.
You might read this and think, that's not really what a sentence does. Fair enough. But let's remember our discussion about words: words acquire meaning in contexts, and as contexts shift, so too do the meanings of words. For our purposes, the motivation to writing a sentence is to express an idea.
Pause for a moment to consider the notion that we can define "sentence" "for our purposes". You'll find, when writing, that paying attention to "your purposes" will help you figure out which words will accomplish what you need, and relevant to this section, will help you figure out how to best express complex ideas.
We can pause on that last statement for a second too. "How to best express complex ideas" suggests there are some standards against which we can compare our expression to some ideal expression, thereby determining whether we have "best expressed" our idea.
However, we do not have a set of standards that tells us how to express particular ideas -- imagine how long that list would be! Instead, we develop a sense of how to express our ideas in a way that our audience will appreciate, understand, get, sympathise with, and so forth. Again, and as with word choice, context is everything; context sets the standards, and so the standards against which we judge our own sentences constantly shift.
You might read this and think, lovely, now I have no idea how to judge whether what I've written is any good or needs work or even makes any sense to anyone other than me. Here's the thing: we share a context -- an environment -- in which we learn words. When I say "pie" in New Zealand, I mean a meat-stew-filled pastry. When I say "pie" in the United States, I mean a sweet, open-top dessert. In the United States, we might have pie for "dessert", but in New Zealand, we have sweets for "pudding". Different contexts call for different words.
Sentences are similar. Some linguistic communities prefer passive constructions, some avoid negative language, and others drop curses roughly every other word. All of this has been a roundabout way of saying that you need to:
Know your audience.
Now, where do ideas come from?
The classic writers block is a seeming inability to generate ideas. You sit down to write, and nothing occurs to you. You think harder and harder about your topic, and still nothing. Where are all the ideas?
Well, most importantly, ideas come from living. If you sit down to write and can't think of anything to write, then by all means stand back up. You'll need to do something to inspire an idea.
For me, many ideas come from art. If I can't think of anything to write, I might visit a gallery, then write about the art in the gallery -- try to figure out the stories the paintings (or sculptures -- I really like sculptures) tell. Typically, for me, generating ideas about other things inspires new ideas that I can then write. And notice, all of this depends on getting out in the world.
Ideas and expressions arise in the world.
Strunk and White, in Elements of Style, call the paragraph the "unit of composition". Given all we've said here, it's fair to call the sentence the "unit of expression". Sentences should be simple; they should express one idea clearly. Paragraph are combinations of simple ideas into more complex forms. It's worth making this distinction now in order to emphasise that we're building our skills from simpler to more complex.
Certainly while you're first learning to write, it's best to keep sentences simple; it's best to pack only one idea into a sentence. Of course, this rule was made to be broken, and on some occasions -- perhaps many occasions, depending on circumstances -- it will be best to express more than one idea in a sentence.
Consider: does a sentence with a semicolon express one idea or two ideas? I won't pretend to answer this question comprehensively. I'll simply note if you consider the question, and how it might be answered, your considerations should help elucidate much of what we're saying here. I might say that the semicolon suggests an alternative expression of the same idea, often for the purpose of clarification. The way you interpret the same might lead you to think that there are, in fact, two different ideas, if there are, in fact, two different expressions. Your mileage will surely vary.
Mechanics
Typically, a sentence has a subject, a verb, and an object. When the subject performs the action, the verb is active. When the object performs the action, the verb is passive. In general, we favour active constructions. (Recall the examples we looked at earlier.)
Avoid vague or ambiguous subjects.
Casually, I might say: "It's best to avoid vague or ambiguous subjects." What is the subject of this sentence? Apparently, "it", which is not a particularly good subject. Casually, we interpret "it" as "you" in this case, which is also the unstated subject of "Avoid vague or ambiguous subjects". In this case, "it's best to" might be better expressed "you should". Then, you're clearly talking to the reader directly; readers appreciate this. So, another way to express this rule is:
Talk to the reader directly.
Notice what we've done here: we've rewritten, essentially, and re-expressed the same idea. So, we can now choose between two expressions, depending on our purposes, as always. This is something you should practise in your writing. You'll find it useful to be able to rephrase and paraphrase for different audiences, and you'll find that practising rephrasing and paraphrasing will help you master your topic, possibly well beyond your expectations.
In the next section, we'll consider more about the form in which we present our ideas. Specifically, we'll consider how the words sound when strung together in sentences -- what we will call "cadence" -- and we'll consider literary devices we can use to make meaningful comparisons between new and familiar ideas.