Why is "speak" a class 4 strong verb?

Question

I've been trying to understand the how strong verbs in Germanic languages work, and reading the Wikipedia article I understand that class 4 strong verbs originated from, in PIE, vowel + a sonorant (m, n, r, or l).

But there seems to be many words whose vowel is not followed by m, n, r, or l but are of class 4. For example, German sprechen and Dutch spreken and English speak is class 4, yet in Old English and Old Dutch the same word seems to be a class 5.

In the article it is noted that

Get, speak, tread and weave were originally of class 5, whereas swear was originally class 6.

How did these verbs "switch" class? And how can a verb like speak become a class 4 verb?

Answer 1

There's a general levelling pattern in the verb morphology of Germanic languages. Wiktionary otherwise mentions "speak" as a strong class 5 verb, primarily due to the fact that it used to be "spake" in the past tense. The reason it became classified as a class 4 verb is that it replaced the older form, "spake", with a (possibly analogous) form "spoke" that matches the class 5 pattern of e/a > o (break/broke, tear/tore, wake/woke etc.); additionally, some other class 4 verbs, like "get" were borrowed.

There was no switching of verb classes when it came to the verbs you mentioned: the Great Vowel Shift basically tore apart class 5 and spread its verbs around after they analogised, likewise with class 6. It's more of a change of terminology than an actual switch of ablaut class. The example you gave, "swear", hasn't really changed that much: in Old English it was swere/swōr and in modern English it's swear/swore. The disintegration of classes 5 and 6 also took place with class 7 and class 2. There's still class 1, class 3 and class 4 strong verbs that remain partially regular and semiproductive in a variety of dialects.

As for other languages' verb classes, it's again due to their vowels' history and entirely unrelated to that of English.