Many English verbs consist of the suffix -en. This suffix usually combines with adjectives, but in certain cases with nouns. Here is a list of verbs:
The suffix -en usually combines with adjectives, but in a few cases the root is a noun. This is the case with strengthen and threaten.
The root never ends in a nasal or in a vowel. For example, the roots of blacken, deepen and deafen all end with consonants. The root often ends with the sound [t] as in bright, flat, fright, light, short, straight, sweet, threat and white.
In many cases only one word in a pair of opposites occur. For example, the word sick combines with the suffix -en to form sicken, but the suffix doesn't combine with healthy. Wide combines with the suffix to form widen, but not with the word narrow.
The colours black, red and white combine with the suffix to form blacken, redden and whiten, but this isn't the case with other colours such as green, pink and yellow.
The suffix -en combines with adjectives and nouns to form verbs. The root always ends in a consonant to the exclusion of nasals. Opposite pairs can be formed such as darken and lighten, but in certain cases such as thicken, only one word in the pair combines with the suffix -en.