Explain constructor rules better in Classes.md

Author: pdfernhout

pages/Classes.md

@@ -71,9 +71,10 @@ tom.move(34);
 ```
 
 This example covers quite a few of the inheritance features in TypeScript that are common to other languages.
+
 Here we see the `extends` keywords used to create a subclass. You can see this where `Horse` and `Snake` subclass the base class `Animal` and gain access to its features.
 
-Derived classes that contain constructor functions must call `super()` which will execute the constructor function on the base class.
+TypeScript has some restrictions about constructors intended to help ensure instances of a class have their properties correctly initialized before the properties are used. Derived classes that contain constructor functions must call `super()` which will execute the constructor function on the base class. You have to call `super()` before accessing `this` in the derived constructor. Both `Horse` and `Snake` technically do that since neither one actually references `this`. A 'super' call must also be the first statement in the constructor of a derived class when the derived class contains initialized properties or has parameter properties. Neither `Horse` or `Snake`, however, have those kind of properties right now. So, as long as those two classes stay that way, they could have console logging or other statements before the `super` call in their constructors without generating compile-time errors.
 
 The example also shows how to override methods in the base class with methods that are specialized for the subclass.
 Here both `Snake` and `Horse` create a `move` method that overrides the `move` from `Animal`, giving it functionality specific to each class.