Green Witches dwell in wild places, exploring the way in which magic weaves in and out of the natural world. Those who are not familiar with such things might at first mistake them for druids, but that is a severe misunderstanding. Green witches do not revere or serve nature itself, but rather study the ways magic interacts with nature, the types of magic drawn to it, and the ways that magic can manipulate natural forces, often with a looser definition of what counts as "natural" than a druid, and blend traditional nature magic with other forms.
When you select this archetype at 2nd level you add the nature spell school to your list of witch spells. You are also under the constant effect of a speak with animals spell.
At 6th level you ignore non-magical difficult terrain and have advantage on saving throws against spells from the plant school.
At 10th level whenever you cast a spell that summons a beast, plant, elemental, or fey, any creatures summoned gain temporary hit points equal to a roll of a number of d4s equal to your proficiency bonus. If multiple creatures are summoned by one spell, they all gain the same number of temporary hit points. They also gain an expertise die on saving throws and attack rolls.
At 14th level, you may cast the shapechange spell once per long rest without expending a spell slot. You may only use this spell to take on the form of a beast, plant, fey, elemental, monstrosity, or giant. At 17th level, you may also assume the form of a dragon.