As the name implies, adjective phrases (AdjP) have an adjective as the head word, e.g.

Adjective Phrase 'happy'

Like noun phrases, the head adjective in an adjective phrase can also receive pre-modification through the use of an intensifying adverb.

Pre-modifying intensifying adverbs

We know that intensifying adverbs are capable of modifying gradable adjectives. For example, quiet is a gradable adjective that can be modified by a preceding intensifying adverb, e.g. very quiet, rather quiet, moderately quiet.

The adjective soft is another example of a gradable adjective that can be modified by intensifying adverbs to produce utterances such as very soft, fairly soft, reasonably soft, and so on.

In addition, intensifying adverbs always precede their associated adjective and the explanation for this should now be apparent, i.e. it is because they function as pre-modifiers in adjective phrases. Here is an example of the use of the intensifying adverb very to pre-modify the head adjective happy.

Adjective Phrase 'very happy'

A further example should suffice to illustrate this usage.

Adjective Phrase 'extremely sad'

This is a fairly straightforward example in which the intensifying adverb extremely is used to pre-modify the head adjective sad.