In Germanic languages, weak verbs are those verbs that form their preterites and past participles of weak verbs by means of a dental suffix, an inflection that contains a /t/ or /d/ sound.

Examples to love - loved
to say - said

The weak conjugation of verbs is an innovation of Proto-Germanic. The dental was probably originally a form of the verb "to do" which was suffixed to the present to form a past tense, just as in English we can form a past tense with "did": he did say.

These should be contrasted with strong verbs, which form their past tenses by means of ablaut. All the original Indo-European verbs which came into Germanic as verbs were once strong. However, as the ablaut system is no longer productive, all new verbs in Germanic languages are weak, and the majority of the original strong verbs have become weak by analogy.

Weak verbs are often thought of as having a regular inflection, but not all weak verbs are regular verbs in English; some have been made irregular by eclipsis or contraction, such as hear ~ heard; while others are merely irregular due to the eccentricities of English spelling, such as lay ~ laid. Some, like bring ~ brought have a vowel change caused by umlaut, sometimes accompanied by a consonant change (think ~ thought). Others, like can or may, the so-called praetero-presentiae , have irregularities caused by the fact that their present tense were originally formed from the past tense of strong verbs, but they later developed a weak formation in the past.

The term "weak verb" was originally coined by Jakob Grimm and in his sense refers only to Germanic philology. However, the term is sometimes applied to other language groups to designate phenomena which are not really analagous. For example, Hebrew irregular verbs are sometimes called weak verbs because one of their radicals is weak.