First, find the stem, which in this case is mendac- (as always, the stem is found by simply removing the genitive singular ending).
Mendax is clearly a third declension adjective, and the dative plural ending for such adjectives is -ibus regardless of gender.
Therefore: mendacibus
To decline the Latin noun "nomen" (name), you would follow the third declension pattern. The nominative singular is "nomen," genitive singular is "nominis," dative singular is "nomini," accusative singular is "nomen," ablative singular is "nomine," nominative plural is "nomina," genitive plural is "nominum," dative plural is "nominibus," accusative plural is "nomina," and ablative plural is "nominibus."
Cirri or cirrus are the latin words which mean curl of hair. The base word form for 'curl' is 'cirrus', and that may be the word you were asking about. 'Cirris' is an inflected form - a changed ending that tells how the word is used. 'Cirris' is either dative or ablative plural, so it would translate to either 'to the curls (dative)' or 'by means of the curls (ablative). Either of those uses would be odd.
The English plural is terrariums. The Latin plural would be terraria.
The genus name is Latin but the English plural would be "diplodocuses."
"You are" would be "es", if it is singular, or "estis", if it is plural.
I suspect it would be Horti
Ferrum = iron The form "ferro" would be in the dative or ablative case, not the nominative one. Dative would be an unusual form, so it's far more likely ablative, specifically ablative of material, best translated as 'made of iron'. Ferrum = Latin word from which the symbol of iron is Fe and such words as ferrous and ferric
The term "pelvimeter" is derived from the Latin "pelvis," meaning "pelvis." In Latin, the plural form of "pelvis" is "pelves." Therefore, the Greek or Latin plural of "pelvimeter" would be "pelvimeters" in English, but if strictly following Latin conventions, it could be referred to as "pelvimetra" when adopting a more classical pluralization.
Use the verb ignoscere to say forgive, since it takes the dative case, the pronoun for "you" would be tibi (or vobis, if the object is plural).Because in Latin the person forgiven is not the direct object of the verb, a literal translation of "you are forgiven" isn't possible. One way around this is to recast the sentence as "I forgive you", tibi ignosco. Another is to use the Latin impersonal passive, tibi ignoscitur, literally "It is forgiven [to] you".
Preces - That's a plural form. Romans used the plural where we would use singular.
The English plural is carcinomas. The Greek/Latin plural would be carcinomata.
Several online dictionaries do not list a plural. One listed phoenixes. But the Latin nominative plural is phoenīcēs (much like the plural of matrix is matrices). So I guess it would depend on if you wanted to retain the Latin.