If a verb is irregular in one tense, will it also be irregular in all other tenses?

Question

For example, if a verb is irregular in the present tense, will it then necessarily be irregular in the present perfect (end in "-en" rather than in "-t" and/or have its root modified)?

Answer 1

To answer the question in the title and not the example, there are verbs that are regular in the present tense but irregular in the past tense. So the answer is no. For example "springen", "er springt", "er sprang", "er ist gesprungen".

In general there are more useful ways to classify verbs other than as just "regular" and "irregular". This is in contrast to English where irregular verbs are less common but you pretty much just have to memorize how they're conjugated in irregular cases. To my mind, the "regular" vs. "irregular" division is another one of those concepts which you learn studying English grammar, but is not as helpful in German. (Another one is "direct" vs. "indirect" object.) English verb conjugation is simpler overall than German, but German makes up for it in that there are many patterns in conjugation, and once you learn to recognize them conjugation becomes much more predictable.

I've found it best to classify verbs into three broad categories: weak, strong, and what I call past-like present. Weak verbs generally follow a single pattern for forming past tenses. For example "sagen", "er sagt", "er sagte", "er hat gesagt". Strong verbs follow a different pattern, including vowel changes, in past tense as in "springen" mentioned above. Sometimes strong verbs are further divided into classes 1-6, but while the class division may help if you're learning Old High German, I don't think it's useful for learners of Modern Standard German. Past-like present verbs are those whose present tense conjugation is similar to the strong preterite pattern, and whose preterite follows a modified version of the weak pattern. For example "wissen", "er weiß", "er wusste". There are only a few of these verbs, but they include all modal verbs so they're very common. (Past-like present verbs are often just called modal verbs, but they are different concepts. "Modal" describes how the verb is used in a sentence, in other words where it appears. Meanwhile "past-like present" describes how the verb is conjugated, in other words how it appears.) There are three verbs, "sein", "tun", and "werden" which are neither strong, weak or past-like present since they follow patterns of their own. Another useful division within strong verbs are stem-changing verbs. These involve a vowel change in present tense, for example "essen", "er isst", "er aß", "er hat gegessen". There are (of course) some verbs which don't fit the patterns completely, such as weak verbs which have a vowel change and/or other irregularity in the past tenses (e.g. "bringen"), but they are few in number and nearly always follow the same pattern as other verbs with only a few exceptions, and even the exceptions follow patterns if you look for them. The most irregular verb in German is "sein", but that's used so often that you won't have any trouble getting enough practice with it.

German dictionaries generally only give the infinitive, 3rd person singular, 3rd person preterite, and past participle. That's because you can almost always determine the entire conjugation table from this information alone. But that's assuming you know all the rules and patterns needed. The good news is that German speakers are very loyal to their conjugation patterns and follow them even with verbs borrowed from other languages. I recently game across "gedownloadet"; the root is "to download" from English, but the participle is formed following German rules, hence "gedownloadet".

Answer 2

Verbs that have a vowel change in the present tense are of the strong conjugation pattern, yes. So their Partizip II is going to end in -en, not -t.

But I recommend to drill infinitive, third person Präsens, third person Präteritum, Perfekt auxiliary, Partizip II for each and every verb, as there are more oddities. Examples:

• verkaufen, es verkauft, es verkaufte, es hat verkauft
• einkaufen, es kauft ein, es kaufte ein, es hat eingekauft.
• verstehen, es versteht, es verstand, es hat verstanden
• aufstehen, es steht auf, es stand auf, es ist aufgestanden
• verlieren, es verliert, es verlor, es hat verloren
• rasieren, es rasiert, es rasierte, es hat rasiert

Better dictionaries show those four forms. You can guess all the other forms of a verb from those four.

Answer 3

There are verbs with weak (regular) present and preterite but a strong perfect participle (Partizip II).

One example is spalten: Es spaltet (present), spaltete (preterite), but perfect participle: gespalten. However, gespaltet is also possible, although it somehow feels wrong to me, may be a regional thing.