When comparing languages, one measure often brought into play as one of the few quantitative statistics for a language is the number of irregular verbs in that language. These counts are not particularly accurate for a wide variety of reasons, detailed in this article.
Classes of verbs in English include:
Other languages have different numbers of regular verbs; for instance, French divides regular verbs into just three categories ("er", "ir", and "re" endings). How many patterns of conjugation are considered standard in a given language is often up for debate. If a large enough group of irregular verbs in a language have parallel conjugations, it is almost arbitrary whether to count that as an additional "standard" conjugation or as a large collection of irregular verbs. (In Spanish, for example, there are nearly as many verbs that conjugate like pensar as those that do so like vivir, and most of the latter type of verbs are very rarely used, yet vivir is always considered regular and pensar irregular.)
In Latin, similarly, most verbs outside the first or fourth conjugations have three principal parts, which form part of the lexicon and must be learned. The three principal parts are the present tense stem, the perfect tense stem, and the past participle; a variety of inflections, ablaut, and sometimes reduplication are used to form these parts. For example, the principal parts of spondeō ("I promise") include spondēre ("to promise"), spopondī ("I promised"), showing reduplication, and sponsus ("promised"); these forms cannot be predicted from the present stem, but when you know all four, the entire system can be constructed from these three parts by rule. This verb is not usually considered irregular in Latin. Latin also exhibits deponent verbs, inflected in the passive voice alone; and defective verbs, missing some principal parts. Truly irregular verbs in Latin are a rather small class; they include esse ("to be"); dare and its derivatives ("to give"); edere ("to eat"); ferre and its derivatives ("to carry"); velle and its derivatives ("to wish"); ire and its derivatives ("to go"); fieri ("to become")and malle ("to prefer"). Most irregular Latin verbs are themselves vestiges of the athematic conjugations of Indo-European, a surviving (and regular) group found in Greek.
Greek and Sanskrit show even greater complexities, with widely different thematic and athematic inflection sets; which set goes with which verb stem cannot be predicted by rule. In languages of this type, these variations are not usually enough to label a verb "irregular". They instead form a part of the lexicon; when a verb is learned, the various patterns used to conjugate it must also be learned.
By contrast, in modern English, the strong verbs are largely a closed and vestigial class. (Analogy has created a few new strong verbs, such as dive.) All of the surviving strong verbs differ markedly from other verbs, and thus are classified as "irregular"; here, they are conspicuous exceptions in the midst of a much larger class of rule-bound regular verbs.
In English, to withhold conjugates exactly like to hold, and in Spanish, detener ("to detain") conjugates exactly like tener ("to have"). In each case, it is questionable if the compound verb and the main verb are both irregular verbs, or as a single irregular verb, with an optional prefix. The question is compounded by the fact that it is not always predictable if the compound conjugates the same as the base. In Spanish, bendecir ("to bless") conjugates almost exactly like decir ("to say"), but there are significant differences in a few tenses that are impossible to foresee.
English has similar cases; the verb "pay" sounds regular: "I pay", "I paid", and "I have paid" are all pronounced as expected. But the spelling is irregular and that cannot be perfectly predicted—for example, "pay" and "lay" turn into "paid" and "laid", but "sway" and "stay" turn into "swayed" and "stayed". For this reason "pay" and verbs like it are almost always considered irregular.