To make clear sentences, you need to think clearly. Don't get confused and say things twice over, e.g., "In my opinion, I think"; or say, "He asked him to marry his daughter," when obviously you don't mean that at all, but, "He asked him whether he might marry his daughter." If you think you will not make statements like this one from a schoolboy's essay: "Very soon clouds of dense fog gather round one on the mountaintop, and one is left to God's mercy, like a bird before the jaws of a snake."

Every word in a sentence serves a specific purpose within the structure of that particular sentence. Sentence structure can be quite complicated, according to rules of grammar. Sentence types can also be combined. A compound-complex sentence contains at least two independent clauses and at least one dependent clause.

Sentences

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Think clearly, and if you have a great deal to say, say it bit by bit, not all in a rush in one sentence. Very long sentences are clumsy and not very clear; e.g., "Owing to the fact that while shaving he had cut his chin, which had necessitated his hunting through all the drawers for a piece of plaster, as a result of which he had lost five precious minutes, and in the haste that had resulted, having spilt some egg down his tie, had had to spend another three minutes washing off the stains, he was late for work that morning, for, having started from home eight minutes later than usual, though he had run all the way, he had not been able to make up the lost time." .....

Types of Sentences

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How much this gains in clarity and ease when it is cut up as follows:
"He was late for his work that morning. No wonder! While shaving he had cut his chin. Then he had lost five precious minutes hunting through all his drawers for a piece of plaster. In the haste that had resulted, he had spilt some egg down his tie, and it had taken three minutes to wash off the stains. Consequently, he had started from home eight minutes later than usual, and, though he had run all the way, he had not been able to make up the lost time."

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Do not deduce from this you should never write a "complex" sentence. A "complex" sentence, may be clearer than a series of "simple" sentences. Besides, a long sentence may give the effect you want better than a series of short ones; and the effect is part of the meaning.

Note, it is very difficult indeed to write on abstract or general subjects. If the subject you set is "Patriotism," for example, turn the essay into a story, or into two or three contrasting stories, illustrating patriotism, or into a debate (this is more difficult) on whether patriotism is a force for good or evil. Or suppose the subject is "Locusts." If you are interested in biology, give a clear account of some interesting scientific facts about the locust, or give a lively description of the first plague in Egypt from the point of view of an Egyptian in the time of Moses, or a graphic account of a swarm of locusts and its effect on the crops on some farms. Whatever you do, don't give facts, opinions and descriptions higgledy-piggledy in dreary confusion!

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Source: English Grammar: EBH Joubert
https://academicguides.waldenu.edu/writingcenter/grammar/sentencestructure

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