Lat. convert- and convers-
A number of these cognates are useful or good friends, in the sense that their meanings are quite similar. Take first the pair Eng. convert ~ Sp. convertir, derived from Lat. convĕrtĕre ‘to turn around, transform, convert’ (con+vĕrt+ĕ+re), derived from vĕrtĕre by means of the prefix con‑ ‘with, together’. The English verb convert [kən.ˈvɜɹt] is a 13th century loanword from Old French, where it was not a patrimonial word. Spanish convertir is also a learned borrowing from written Latin, perhaps through French, also from the 13th century.
English has a noun also written convert, but pronounced [ˈkɒn.vəɹt], which means ‘someone who has been persuaded to change their beliefs and accept a particular religion or opinion’ (DOCE). This noun, which appeared in English in the 16th century, seems to be a back-formation of the verb to convert, i.e. the noun was derived from the verb, in English itself (cf. Part I, §5.9). As we saw in the preceding section, the equivalent noun in Spanish is converso/a, a noun that is a loanword from the passive participle of Latin convĕrtĕre, namely conversus ‘turned around’ (con+vers+us; fem. conversa). (This passive participle was converted to a noun in Latin, but only with the meaning ‘a turning or twisting around’.)
As we mentioned earlier, all the derived Spanish verbs on this list are third conjugation ‑ir verbs, unlike verter, which is a second conjugation ‑er verb. As we have seen before, Latin third conjugation ‑ĕre verbs became either second ‑er or third ‑ir conjugation verbs in Spanish. Here we see how both options were realized with this very same root. Also interesting is that even though these are learned verbs, the stem vowel e, from the original Latin short ĕ, behaves just like patrimonial vowels do in that it diphthongizes to ie when stressed, as in yo convierto ‘I convert’. That is not the norm with learned verbs that are borrowed from written Latin and the reason must have had something to do with the influence (analogy) of verter, which is a patrimonial verb.
From the very same stem convers‑ (con+vers‑), Latin created other words, such as the noun conversiōnem (nom. conversiō), for ‘the act of converting’, formed with the suffix ‑iōn‑. This noun has given us the cognates Eng. conversion [kən.ˈvɜɹ.ʃən] and Sp. conversión [kom.beɾ.ˈsi̯on].
From the same stem convers‑, Latin derived the verb conversāre, a frequentative version of convĕrtĕre, which came to have the meanings ‘to turn around or over’, ‘to ponder’, and ‘to consort or associate with’. The passive version of this frequentative verb was conversārī, which had various meanings such as ‘to occupy oneself’, ‘to keep company (with)’, and ‘to live with’. This verb was borrowed by English and Spanish, giving us the verbs Eng. converse [kən.ˈvɜɹs] and Sp. conversar [kom.beɾ.ˈsaɾ]. Both cognates have the same meaning, namely ‘engage in conversation’ (COED), and they both appeared around the same time, namely 16th century, perhaps under French influence, since French borrowed the verb converser from Latin in the 11th century with the meaning ‘to live somewhere’, which changed in the 12th century to ‘to live with someone’, and the meaning ‘engage in conversation’ is not attested until the late 17th century.
The Latin noun derived from this Latin verb was conversātĭo, which meant ‘frequent use’, ‘frequent abode in a place’, and ‘intercourse, conversation’. Eng. conversation ~ Sp. conversación have been borrowed with the latter of these meanings. Note that English also has an noun converse that means ‘a situation, fact, or statement that is the opposite of another’ (COED), which can also be an adjective meaning ‘opposite’, as in the converse situation. This adjective does not have a Spanish cognate, and it translates as opuesto/a or contrario. The noun converse, which is always preceded by the definite article the, translates as lo opuesto, lo contrario.
From the stem convert‑ we have the cognate adjectives Eng. convertible and Sp. convertible, both of which mean ‘that can be converted’. Both ultimately from Latin convĕrtĭbilis ‘changeable’ (con+vĕrt+ĭ+bĭl+is), formed with the adjectival suffix ‑bĭl‑ (cf. Eng. ‑able/ible). It is likely that French borrowed this adjective first and then passed it on to Spanish and English. English convertible [kən.ˈvɜɹ.ɾə.bəl] has been around since the 14th century and it can be used as a noun since the 17th century, especially since the early 20th century for an automobile with a detachable top, short for convertible automobile (Sp. descapotable). Other English examples of uses of this adjective are convertible sofa (it can be turned into a bed, cf. Sp. sofá cama), convertible currency (it can be converted to other currencies, cf. Sp. moneda convertible), convertible collar (it can be used with the neck button fastened or unfastened).
There are a few other words derived from the stems the convert‑ and convers‑. One of them is Eng. converter (convert+er), also spelled convertor (convert+or), literally meaning ‘thing that converts’. This word was formed in English out of the verb convert and the agentive suffix ‑er or ‑or. It has several different uses, one of them, for example, is ‘electronic device that converts one frequency of a radio signal to another’ (AHD). It typically translates into Spanish as convertidor (con+vert+i+dor), derived from the verb convertir and the Spanish agentive suffix ‑dor, e.g. convertidor de corriente eléctrica ‘electric current converter’ or convertidor de señales de televisión ‘TV signal converter’. Another option in Spanish is conversor, as in analogue-to-digital converter, which in Spanish is sometimes called conversor de señal analógica a digital. This conversor also does not come from a Latin word, but it is formed from the stem convers‑ and the agentive suffix ‑or, by analogy with other agentive nouns derived from prefixed vĕrtĕre verbs. Sometimes a converter/or can also translate as transformador (cf. Eng. transformer).
 A convertible (car) is known in Spanish as un descapotable, a borrowing from French décapotable (1927), a noun related to the verb décapoter ‘to lower the top of’ (1929), derived from the feminine noun capote ‘greatcoat, bonnet, car top’ (cf. Sp. capota ‘folding cover’), related to the masculine capot ‘cover, hood’ (bonnet in British English) (cf. dialectal Sp. capó ‘car hood’, also known as cofre in some Spanish-speaking countries).