It in turn divided into North, West and East Germanic groups, and ultimately produced a large group of mediaeval and modern languages, most importantly: Danish, Norwegian, and Swedish (North); English, Dutch and German (West); and Gothic (East, extinct).
There is one verb (*dōną 'to do') that is in a category of its own, based on an Indo-European "athematic" form, and having a "weak" preterite but a "strong" passive participle.
An example in modern Dutch is verliezen – verloor – verloren (to lose) These are the direct descendants of the verb in Proto-Indo-European, and are paralleled in other Indo-European languages.
Because the perfect in late Indo-European was no longer simply stative, but began to be used especially of stative actions whose source was a completed action in the past (e.g., Greek), this anterior aspect of it was emphasized in a couple of Indo-European daughter languages (e.g., Latin), and so it was with Germanic that the perfect came to be used as a simple past tense.
An example of this is the typical and widespread PIE stative *woida 'I know': one who "knows" something at some point in the past "came to know" it, much as the natural inference from noting someone in a sitting state is that a prior action of becoming seated occurred.
Apparently it was this latter anterior respect that is responsible for the Indo-European perfect showing up as a past tense in Germanic, Italic, and Celtic.
The Indo-European perfect originally carried its own set of personal endings, the remnants of which are seen in the Germanic strong preterite.
Kann "can, am/is able to" (present tense) displays the vowel change and lack of a personal ending that would otherwise mark a strong preterite.
[1]: 174–175 This approach allows the preterite-presents to be treated as purely stative in origin without depriving the PIE perfect of a temporal element.
The known verbs in Proto-Germanic (PGmc): The present tense has the form of a vocalic (strong) preterite, with vowel-alternation between singular and plural.
The root shape of the preterite (in zero-grade) serves as the basis for the infinitive and past participle, thus Old English infinitive witan and past participle (ge)witen; this contrasts with all other Germanic verb types, in which the basis for those forms is the present stem.
In modern English, preterite-present verbs are identifiable by the absence of an -s suffix on the third-person singular present tense form.
Compare, for instance, he can with he sings (preterite: he sang); the present paradigm of can is thus parallel with the past tense of a strong verb.
The early history of will (German wollen) is more complicated, as it goes back to an Indo-European optative mood, but the result in the modern languages is likewise a preterite-present paradigm.
The copula (the verb to be and its equivalents in the other languages) has its forms from three or four IE roots (*h₁es-, *bʰuH-, *h₂wes-, and possibly *h₁er-.