Something happened and you want to write about it. You put some first thoughts down and then you realize that the same scene, actual or imaginative, can be observed from more than one angle, that you can change the way your reader sees what you are depicting. How can a little grammatical knowledge about sentence construction help you make these changes more insightfully?
Let’s say you want to write about two friends who have not seen each other in a good number of years. One visits the other, and they go for a walk. In your first draft you write They walked and they talked. One sentence, two independent clauses, and the effect you create with those two simple and complete declarations is of two clear strong lights shining directly on each of two happenings. Their walking and their talking are balanced, and you give your reader the impression, unconscious of course, that both events are of equal importance.
How would the picture change, though, if you omitted the subject of the second clause, taking advantage of the rhetorical device called ellipsis: They walked and talked. Now the two clear lights that you shined on the two independent clauses in your first version have melded into one, concentrating your reader’s attention on one subject, they, who did two things, walked and talked. With that change in distribution, you have sped up the pace of your sentence; it moves more quickly because with the omission of the second subject, it is one degree less precise; a unit of two is now acting, not two distinct and single units. That change might work better for the circumstances you want to portray—and it might very well not. That’s for you to decide.
But there are other revisions yet to consider. How does the picture change if you subordinate the first clause: While they walked, they talked. To subordinate a clause means to put it in the background of the scene. Because a subordinate clause is neither grammatically nor logically independent in its own right (to just say while they walked would leave someone waiting for you to complete your thought), the light you are shining on the scene changes even more. Now only one event, the independent clause they talked, is put brightly before the reader’s attention, with the other action, they walked, seen more dimly as the circumstances in which this one main action prominently occurred. Invert the order of the two clauses, They talked while they walked, and you change the balance again, because the final position of an English sentence is the most emphatic. If you decide instead to change the clause you subordinate, you give yourself two more possibilities by which to present the moment to your reader: While they talked, they walked or They walked while they talked.
And one more. Subordinate clauses can be reduced to participial phrases, which compact an assertion into a concentrated statement. Instead of while they walked, we can write simply the present participle walking: Walking, they talked. Or, of course, Talking, they walked. All of these revisions alter the way in which your reader sees what you have to say, and the point of understanding the grammar behind each variation is to put those many possibilities consciously into the back of your mind. By once becoming conscious of how something is done, it can become a directing second nature, and you’ll have reach and recourse to possibilities once unrealized. That, in turn, can clear away obstacles and frustration both, and the attending freedom can make the difficult task of writing one step more enjoyable.