What are verbs?
Verbs are words that express actions, processes, or states of being. The verb form that you will find in your vocabulary list and in dictionaries is called the infinitive. While in English the infinitive of a verb consists of to plus the verb (to give, to love, to be), infinitives of all German verbs end in -en or (less commonly) -n (geben, lieben, sein).
Verbs in German are conjugated, i.e. their endings are changed, to agree with the sentence subject. Other grammatical information, such as tense, mood, and voice, is conveyed through additional changes to verb forms. These changes include further suffixes or vowel changes and sometimes the additional use of an auxiliary verb.
Every German verb has three principal parts: the infinitive, the third person simple past, and the past participle. For example, the three principle parts of trinken are trinken, trank, getrunken (compare English drink, drank, drunk). If you know the principle parts of a verb, you typically have all the information you need about the word to conjugate it in any tense in German.
Weak, strong, and mixed German verbs
Depending on how they form their principal parts, German verbs are classified as weak, strong, or mixed. The infinitive form contains no indication of whether a verb is weak, strong, or mixed. The differences among these verbs are most apparent in the other two principle parts, and thus also in the tenses that use these parts.
German weak verbs are similar to English regular verbs; there is no stem change and they follow predictable patterns in all forms and tenses: spielen, spielte, gespielt (play, played, played); fragen, fragte, gefragt (ask, asked, asked); glauben, glaubte, geglaubt (believe, believed, believed). The vast majority of verbs in German are weak verbs.
German strong verbs always undergo a stem vowel change in the simple past forms and often in the past participle as well, as is the case with many strong (irregular) verbs in English: singen, sang, gesungen (sing, sang, sung); sprechen, sprach, gesprochen (speak, spoke, spoken); kommen, kam, gekommen (come, came, come). There are a limited number of strong verbs in German, but a great number of them are frequently used words.
German mixed verbs undergo a stem-vowel change, as with strong verbs, but the suffixes and personal endings correspond to those of the weak verbs: bringen, brachte, gebracht (bring, brought, brought); denken, dachte, gedacht (think, thought, thought). There are just 16 mixed verbs in German, but many of them are fairly common.
Verb tenses, moods, and voices in German
German has 6 different verb tenses, each of which is based on one of the three principal parts. There are two finite or one-word tenses: the present tense and the simple past tense. The remaining four tenses – present perfect, past perfect, future, and future perfect – are compound tenses, meaning that they are formed using a principal part of the main verb plus one or more auxiliary verbs.
Verb tenses indicate the time at which the action, process or state occurs. In addition, German has three moods – indicative, subjunctive, and imperative – and two voices – active and passive. The grammatical category of mood indicates whether stated information is factual or hypothetical or is a command. Voices emphasize a particular relationship between the action and its subject and objects. Most of the time, we use verb tenses in the indicative mood (to state fact) and the active voice (to denote what the action the subject is doing). Thus, when speaking of verb tenses without reference to mood or voice, the tenses referred to are in the indicative mood and active voice.