What is a paragraph? I'll answer that question by quoting from page 328 of the HARBRACE COLLEGE MANUAL (Harcourt Brace Jovanovich, 1941, 1946, 1951, 1956, 1962, 1967, 1972): "A paragraph is a distinct unit of thought -- usually a group of related sentences, though occasionally no more than one sentence -- in a written or printed composition. The form of a paragraph is distinctive: the first line is indented. The content of a unified paragraph deals with one central idea. Each sentence fits into a logical pattern of organization and is therefore carefully related to other sentences in the paragraph."
I'll next quote the opening paragraphs of three or more of Poul Anderson's books to demonstrate examples of especially striking and effective opening paragraphs.
"There was a man called Orm the Strong, a son of Ketil Asmundson who was a great landsman in the north of Jutland. The folk of Ketil had dwelt in Himmerland as long as men remembered, and were mighty landowners. The wife of Ketil was Asgerd, who was a leman-child of Ragnar Hairybreeks. Thus Orm came of good stock, but as he was the fifth living son of his father there could be no large inheritance for him."
The chief point of interest in the text quoted was how Poul Anderson modeled it on the Norse sagas. The strong genealogical orientation should be noted. Also, the story develops from the fact Orm could not hope for a large share of his father's estate. And of the means Orm chose for remedying that.
"It pleased Ruethen of the Long Hand to give a feast and ball at the Crystal Moon for his enemies. He knew they must come. Pride of race had slipped from Terra, while the need to appear well-bred and sophisticated had waxed correspondingly. The fact that spaceships prowled and fought fifty light years beyond Antares, made it all the more impossible a gaucherie to refuse an invitation from the Merseian representative. Besides, one could feel delightfully wicked and ever so delicately in danger."
This paragraph arouses many questions and thoughts. Who exactly were Ruethen and the Merseians? Why did he subtly mock his enemies by giving them a feast and ball? Where and what were the Crystal Moon? Why had "pride of race" slipped from Terra? The comment about fifty light years beyond Antares suggests enormous distances. And a sense of decadence is suggested for Terrans. The paragraph is designed to entice readers to continue so they will find the answers to these questions.
"The story is of a lost treasure guarded by curious monsters, and of captivity in a wilderness, and of a chase through reefs and shoals that could wreck a ship. There is a beautiful girl in it, a magician, a spy or two, and the rivalry of empires. So of course -- Flandry was later tempted to say -- it begins with a coincidence."
This paragraph also inspires many thoughts. The paragraph gives a good cryptic summary of the entire book without giving away details. What lost treasure was guarded by curious monsters? Who were the spies, the beautiful girl, the magician, and the rival empires?
It is my opinion that the three examples quoted above are excellent specimens of Poul Anderson's skill in writing opening paragraphs. Consciously or not they make the readers ask questions and lures them on to read further.