"Ko" is used for "is" or "are" when identifying something. It's often used in a phrase like "Ko wai koe?" (Who are you?).
"He" is used for "is" or "are" when describing something. For example, "He pai te rangi" (The weather is nice).
"Kei" is used for "are" when indicating location. For example, "Kei te whare ahau" (I am at the house).
"E" is used for "are" when describing an action. For example, "E haere ana ahau" (I am going).
The best way to choose the right word depends on the specific sentence you are trying to construct.
If you can give me more context about the sentence you want to use, I can give you a more specific answer.