<p>I may not be 100% correct but this is how I view it.</p>
<p>When you have a “not only but also” problem, the first part of the sentence will be present tense and the second part will be past tense. For example, “Not only did I study for the SAT, but also studied for the ACT.” </p>
<p>Similarly, the Eqyptians “know” in the first part and “knew” in the second part. </p>
<p>“Did . . . study” etc. is still in the past. Perhaps dubnation refers to the auxiliary + infinitive construction, since an infinitive resembles the present tense but is never conjugated.</p>
<p>I think the biggest rule is that the clause (part of the sentence) that goes with “not only” has to match the clause for “but also.”</p>
<p>Not only “did the ancient Egyptians know about the North Star” --> A verb clause (contains the verb “did”)
But they also “knew in precisely which direction it lay” --> Also a verb clause</p>
<p>Now, if you wrote, “Not only did the ancient Egyptians know about the North Star but also about embalming,” that would be wrong because “about embalming” has no verb; it is a noun clause. </p>
<p>To fix it, you would write:
“The ancient Egyptians knew not only about the North Star but also about embalming.” Note that “not only” goes after the verb “knew” in this case.</p>
<p>Also, when “not only” goes before the subject (“the ancient Egyptians”), you usually need a comma and “but” (or a semicolon), followed by another subject in the next clause (“they”). It is wrong to say, “Not only did the ancient Egyptians know about the North Star, but also knew in precisely which direction it lay.” </p>
<p>Other reasons why this is correct:
It is bad form to put a preposition at the end of a sentence (“but they also knew precisely which direction it lay in” is wrong)
“Lay” is the past tense of “lie,” which in this sentence means “to rest,” or “to be in a specific place.” “Laid” is the past tense of “lay,” which means “to put something down,” like you lay a baby in his crib. So “but they also knew precisely in which direction it laid” is wrong.</p>