The suffix
-en, or in the oldest recorded stages of the language
-an, once marked the infinitive of
all verbs. It also marked the past participle of strong verbs, as it still does occasionally. The past participle, however, also had a prefix
y-, in the earliest stages
ge-, which survives in a few relic words beginning with
a- like
adrift,
agog,
awoken.
By about 1500, the suffix had been dropped in just about all cases.
The few exceptions were mostly verbs of colouring (
to whiten,
to blacken,
to redden), and the verb
(a)waken. And a few others as in the lists above.
The -en in these verbs stopped being an infinitive suffix and was absorbed into the stem. Thus
I whiten,
I have whitened, not "*I white", "*I have whited". The original form of the past participle does survive in the biblical phrase "a whited sepulchre", but it is an obvious archaism.
The exception is "waken". Here the Middle English forms were something like"to waken", "I wake", "I woke", "I have awaken" (actually "ywaken", I suppose, pronounced almost like "awoken".)
But in this verb all the permutations with a- and -en have survived:
To wake, waken, awake, awaken.
I wake, awake.
I woke, awoke, wakened, awakened.
I have woken, awoken, wakened, awakened.
PS. I will leave the entry as I wrote it originally, but I was wrong regarding the past participle and the etymology of awake. The prefix a- is "on" really, not the vanyshed y-/ge- of the pasts participle. And here are links for etymologies of
awake, awaken (near the bottom of the page) and
wake, waken (also near bottom). It's rather more complex than I wrote, but it seems the confusion between to (a)waken and to (a)wake happened in part exactly because the -en suffix had marked the infinitive, as I wrote, and dropped out during the 15th century.