Source: p 78, Syntax, A Generative Introduction (3 ed, 2012) by Andrew Carnie.
Though I am only on Chapter 4 at the time of this post, I cannot wait until Chapter 6 to understand the following.
Here is a common mistake to avoid: Notice that the AdvP rule specifies that its modifier is another AdvP: AdvP ⟶ (AdvP) Adv. The rule does NOT say *AdvP ⟶ (Adv) Adv, so you will never get trees of the form shown in (28) [see above]:
You might find the tree in (27) a little confusing. There are two Advs and two AdvPs. In order to understand that tree a little better, let’s introduce a new concept: heads. We’ll spend much more time on heads in chapters 6 and 7, but here’s a first pass: The head of a phrase is the word that gives the phrase its category.
I reread pp 78-79, but still do not understand why 28 is wrong and only 27 is correct.
What is the main idea here? How does the concept of Heads resolve?