1.72Non-finite forms are such as are based on verbs, but do not show distinction of person and number and belong to other than verbal word classes. Verbs normally have:
1.Gerund—an adverb formed by adding -ando to the root of I-conjugation verbs; -endo to roots ending in -y, -ll, -ñ; and -iendo to others: cantando, aprendiendo, viviendo, viendo; oyendo "hearing" (oír "hear"), bullendo "boiling" (bullir "boil"), riñendo "wrangling" (reñir "wrangle").
2.Past participle—an adjective formed by adding -ado to the root of I-conjugation verbs, -ido to the root of II-and III-conjugation verbs, and -do to the root of IV-conjugation verbs. Irregular past participles are all those that have other endings than these, as: abierto "opened", cubierto "covered", dicho "said", escrito "written", hecho "done", impreso "printed", muerto "dead", provisto "provided", resuelto "resolved", roto "broken", visto "seen", vuelto "returned".
3.Infinitive—a masculine singular noun ending in -r. Occasionally an infinitive is used in the plural, which then ends in -es.
1.73Irregular verbs.A number of verbs vary in one way or another from the patterns set forth above. Some of them fall into regular patterns of irregularities:
1.Radical-changing verbs are such as have the regular endings of the conjugation to which they belong, but have certain changes in the vowel of the last syllable of the root, depending on the form involved. There are three main types of radical-changing verbs:
a)Type I, in which a diphthong is substituted for a vowel (ie for e or i; ue for o or u) when it is stressed. (At the beginning of a verb, the sounds ie are spelled ye, and the sounds ue are spelled hue.) These forms are (with contar "count" and pensar "think" as examples):
| Present | 1. | sg | cuento | pienso |
| 2. | sg | cuentas | piensas | |
| 3. | sg | cuenta | piensa | |
| 3. | pl | cuentan | piensan | |
| Present Subjunctive | 1. | sg | cuente | piense |
| 2. | sg | cuentes | pienses | |
| 3. | sg | cuente | piense | |
| 3. | pl | cuenten | piensen | |
| Imperative | 2. | sg | cuenta | piensa |
b)Type II, in which ie is substituted for e, and ue for o, in all forms where stress falls on this vowel, and i is substituted for e, and u for o, wherever the root is unstressed but followed by a, ie, or ió.
c)Type III, in which i is substituted for e both in forms where the last syllable of the root is stressed and where the root is unstressed but followed by a, ie, or ió.
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