One of the most common areas for irregularity in Romance language verbs is the past participle. In English, the regular pattern for forming the past tense is the inclusion of an "-ed" or "-d" at the end of the verb -- the latter in the case of a verb already ending in an "-e." An example is the verb "to open," whose past participle is "opened." Irregular verbs often have unique past participles, as in the case of "think," whose past participle is "thought," rather than "thinked."
A second category of irregular past participles includes those in which the present tense and past tense mirror each other. An English verb that illustrates this type of irregularity is "to quit." The present tense follows the regular pattern of "I quit," "you quit" and "he/she quits," yet the past participle is also "quit," as in, "I quit." The only difference between the past and present tense in these cases is the inclusion of an "-s" in the third-person singular present.
Irregularity in verbs isn't restricted to the usage of their past tenses. Although less common in English, a significant area of verb irregularity is in the conjugation of the present tense. The regular pattern for an English verb follows closely the infinitive, such as with "to dance," which conjugates "I dance," "you dance," "he/she dances," "we dance," "you dance (plural)" and "they dance." In contrast, the English verb "to be," conjugates "I am," "you are," "he/she/it is," "we are," "you are (plural)" and "they are." Unlike a regular verb, no rule applies to its conjugation -- the form must be memorized.
The last form of irregularity often involves the differences between written and spoken language. Many verbs, while retaining the same sound as a regularly conjugated verb, will have distinct spelling changes. For example, the verb "to study" conjugates in a form that phonetically matches the regular pattern of English verbs -- the third-person plural of the verb replaces the "-y" at the end of the word with an "-ies," as in "I study," "you study" and "he/she studies." If the verb were regular, it would conjugate as "studys."