The Weak verbs
In this series, a "weak verb" is defined as: "a verb with a root that has as its radicals: one, and only one, guttural, plus two different strong consonants."
In this instance, resh is considered to be a strong consonant, but waw and yod are, with a few exceptions, not considered to be strong consonants.
The Roman numbers I, II, III, are used to indicate the position of the guttural in the root of the verb. For example: III-hey indicates that the verb has a hey as a radical in the third position. In other words, the root ends in a hey.
The weak verbs can be classified into twelve classes according to the four stronggutturals, multiplied by the three root positions in a root where they could be found. Except for II-guttural verbs, resh is excluded as it presents no unexpected influences.