Chang (常, the Constant) in the Dao De Jing: Meaning and Occurrences
In the Dao De Jing, chang (常) means constant or eternal: the eternal Way of chapter 1, and the constant that chapters 16 and 55 say one should know.
常 (cháng) means lasting, constant, always so. This site translates it "eternal" in chapter 1, "constant" in chapters 16 and 55, and "always" or "constantly" where it works as an adverb. This page lists the uses where 常 means "the constant"; where it only means "always", it is left out.
The book opens with it: the way that can be followed is not the eternal Way (常道), and the name that can be named is not the eternal name (常名).
In chapters 16 and 55, 常 is a noun: knowing the constant is called clarity (知常曰明), and chapter 16 adds that not knowing the constant and acting arbitrarily is harmful. Other set phrases are 常德, the enduring virtue of chapter 28, and 常心, the fixed mind that the sage does not have in chapter 49.
Every occurrence of 常
Chapter 1
Chapter 16
Chapter 28
Chapter 46
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Line 5
故知足之足，常足矣。
So the contentment of knowing what is enough is lasting contentment.
Chapter 49
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Line 1
圣人无常心，
The sage has no fixed mind of his own;
Chapter 52
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Line 11
无遗身殃，是为习常。
bring no harm upon yourself: this is called following the constant.
Chapter 55
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Line 8
知和曰常，知常曰明。
Knowing harmony is called the constant; knowing the constant is called clarity.