The subject of history of English

19. Old English Verbs. Preterite-Present Verbs.
A few Old English verbs (unfortunately they are important and rather common) combine features of Strong Verbs and Weak Verbs.
These verbs take what would normally be a Strong Verb past tense and transfer it to the present. They then build a Weak Verb paradigm upon that Strong Verb present tense.
This sounds confusing, but makes sense when you see it applied to an actual verb. The basic idea is that preterite-present verbs are Strong Verbs that have their past tenses and present tenses swapped.

The important verbs in this category are:

witan = to know
agan = to possess
dugan = to achieve
cunnan = to know
durran = to dare
To construct a conjugation for a Preterite Present Verb, do the following:
Subtract the "an" ending from the infinitive. This gives you the stem of the verb:
witan -"an" = wit
Use the Strong Verb Paradigm to determine what the Past Singular would be:
"wit" would be a Class I Strong Verb, so we know that the Preterite would be "wat"

wit ==> wat

This now becomes the stem for the paradigm, and what you would have expected to be the present tense (wit, which, remember, is the stem minus the "an" ending of the infinitive) moves to the past tense.