The most important word in the game. Noun IS property will give that property to the named object as long as a rule remains intact.
Noun1 IS Noun2 will transform all instances of the first Noun into the second. This happens instantaneously and is not undone if the rule is disrupted. The transformed objects do not retain any traits of what they were transformed from (so Baba Is You / Baba Is Crab will freeze the level unless Crab Is You).
Noun IS Noun (where both nouns are the same) will prevent transformations of the Noun. This can override any transformation of the Noun, but it cannot undo past ones.
Combined with IS, this prevents an object from gaining a particular property. If Noun IS NOT Property, then the Noun can never have that property even if another rule would provide it. One NOT will override any number of rules that provide the property, including ones built into levels.
Noun1 HAS Noun2 will cause any noun1 that is destroyed to spawn a noun2.
Connects together two noun phrases or two property phrases into a compound that applies both. For example, Noun AND Noun IS Property, or Noun IS Property AND Property.
Noun1 ON Noun2 behaves like a single Noun but applies only to instances of Noun1 that overlap Noun2 at the moment they do so.