In Germanic languages, weak verbs are those verbs that form their preterites and past participles by means of a dental suffix, an inflection that contains a /t/ or /d/ sound or similar. (For comparative purposes, they will be referred to as a dental, but in some of the languages, including most varieties of English, /t/ and /d/ are alveolar instead.) In all Germanic languages, the preterite and past participle forms of weak verbs are formed from the same stem.:
Historically, the pronunciation of the suffix in the vast majority of weak verbs (all four classes) was [ð], but in most sources discussing Proto-Germanic, it is spelled ⟨d⟩ by convention. In the West Germanic languages, the suffix hardened to [d], but it remained a fricative in the other early Germanic languages (Gothic and often in Old Norse).
In English, the dental is a /d/ after a voiced consonant (loved) or vowel (laid), and a /t/ after a voiceless consonant (laughed), but English uses the spelling in ⟨d⟩ regardless of pronunciation, with the exception of a few verbs with irregular spellings.
In Dutch, /t/ and /d/ are distributed as in English provided there is a following vowel, but when there is no following vowel, terminal devoicing causes the pronunciation /t/ in all cases. Nevertheless, Dutch still distinguishes the spellings in ⟨d⟩ and ⟨t⟩ even in final position: see the 't kofschip rule.
In Afrikaans, which descends from Dutch, the past tense has fallen out of use altogether, and the past participle is marked only with the prefix ge-. Therefore, the suffix has disappeared along with the forms that originally contained it.
In German the dental is always /t/ and always spelled ⟨t⟩ because of the third phase of the High German consonant shift (d→t).
In Low German, the dental ending of the preterite tense was originally /d/ or /t/, according to the stem of the verb. However the ending has fallen out in pronunciation, starting in the 17th century when the preterite was written with the ending -er representing the sound [ɐ] which was already the last remnant of the former -de and -te endings of Middle Low German. Now, the only Low German verbs that still show a remnant of a dental ending are leggen, which has the preterite leed, and the verb hebben, which has harr with old r-ending from the Middle Low German dental.
In Icelandic, the dental was originally a voiced dental fricative /ð/. It is preserved as such after vowels, voiced fricatives and /r/ but has been hardened to a stop /d/ after nasals and /l/, and has been devoiced to /t/ after voiceless consonants and in some other cases (in most Old Norse texts, the alternation is already found in heavy roots, but the light ones preserve /ð/). Furthermore, the voicing contrast between /d/ and /t/ has been replaced in Modern Icelandic by an aspiration contrast, which may not be realized phonetically in all the relevant positions.
The situation of early Norwegian was similar to Icelandic, but intervocalic /ð/ eventually disappeared. In the verbs in which it remains, the dental is /t/, /d/, depending on conjugation class and dialect. It is spelled accordingly. In Nynorsk, it can be different in the preterite and the past participle.
Swedish has a similar situation to that of Norwegian, but the dental is retained in the spelling, even between vowels. Some informal spellings indicate a lost dental, such as in sa ("said") from the standard spelling sade.