Adjectives/Adverbs/Conjunctions?

Hello,

I'm reading about adverbial clauses and conjunctions, and the following discussions confuse me:

The book says:
These conjunctions frequently begin adversial clauses that indicate degree or comparison: as, than, etc.

My relatives arrived earlier than they usually do.

In this example, the subordinate conjunction "than" connects the adverbial clause "than they usually do" to the main clause "my relatives arrived earlier."

(Ok, so far, so good. This, I get. But then the books says:)

The adverbial clause modifies the adjective "earlier."

Huh? Why is "earlier" an adjective?
"Arrived" is a conjugated verb. Arrived...when? "Earlier." So, shouldn't "earlier" be an adverb in the main clause, "My relatives arrived earlier." Don't adjectives modify nouns and pronouns?

Then, in another section, the book says:

Most adverbial clauses modify verbs. However, adverbial clauses of comparison often modify adjectives and adverbs.

This computer is faster than that one is.

The subordinate conjunction "than" connects the adverbial clause "than that one is" to the main clause "this computer is faster." The adverbial clause modifies the adjective "faster." It compares the speed of this computer to the speed of that computer. In doing so, the adverbial clause specifies the nature of the comparison made by the comparative adjective faster.

(Ok, I get this, too. But, in the same section, the book says:)

The plane landed later than we expected it to land.

The subordinate conjunction "than" connects the adverbial clause "than we expected it to land" to the main clause "the plane landed later." The adverbial clause modifies the adverb later.

(Fine, This, I get. "Landed" is a conjugated verb. "Landed" when? "later." So, later is an adverb.")

But Im confused, nonetheless. Why is "later" here an adverb, but "earlier" (above) is classified as an adjective?

English is confusing....