-ami / -ama

While -ama is considered highly informal for regular nouns, there is a crucial exception: the historical dual number. For four paired body parts—ruka (hands), noha (legs/feet), oko (eyes), and ucho (ears)—the endings -ama / -ima (rukama, nohama, očima, ušima) are the strictly codified Standard Czech forms. You must use them even in formal writing! (Note that for their diminutives, like nožičky or očička, the dual ending -ama (nožičkama, očičkama) is also mandatory in Standard Czech when referring to actual body parts.) Notice the consonant mutation in the stem for eyes and ears (oko → oč-, ucho → uš-). These strictly take the -ima ending (očima, ušima); forms like *okama, *uchama, or the common learner mistakes *očama and *ušama are incorrect. Also note that while the nominative plural of ruka is ruce, the instrumental dual reverts to the original k-stem (rukama, not *rucema or *rucama). Crucially, all adjectives, pronouns, or numerals modifying these body parts MUST also take the -ma ending in Standard Czech. Hard adjectives take -ýma (e.g., s holýma rukama) and soft adjectives take -íma (e.g., s cizíma očima). This includes pronouns (e.g., Díval se na mě těma velkýma očima or Viděl jsem to svýma vlastníma očima). Note that the pronoun všechen (all) does NOT have a standard dual form; its standard instrumental plural is strictly všemi in all contexts (e.g., před všemi očima). Be aware that the highly common idiom beru to všema deseti (I'll take it eagerly / literally: with all ten fingers) uses the colloquial -ma ending, as the strictly codified Standard Czech form is brát všemi deseti. Note that the numerals dva (two) and oba (both) have the mandatory surviving dual forms dvěma and oběma. These are the only standard instrumental forms of these numerals, and they are used with all nouns, regardless of whether the noun is a body part or not (e.g., s oběma rukama, but also hrnec se dvěma uchy). However, the numerals 3 and 4 change to třema and čtyřma ONLY when modifying these specific body parts (e.g., pes se čtyřma nohama, or the common idiom mezi čtyřma očima meaning 'face to face' / literally 'between four eyes').

However, this dual exception only applies to actual human or animal body parts. When these words refer to inanimate objects, they take regular standard instrumental endings (e.g., stůl se čtyřmi nohami for a table with four legs, s oky for nets/snares/grease drops, s uchy for pot handles, and s ručičkami for clock hands). Also, be careful not to overgeneralize this rule: other paired body parts like koleno (knee), rameno (shoulder), prso (breast), and záda (back) do NOT use the -ma ending. They take regular standard instrumental endings (s koleny, rameny, prsy, se zády).

When speaking casually about regular nouns, be careful not to mix registers. If you use a colloquial noun ending like -ama, the attached adjectives and pronouns must also use colloquial endings (e.g., s těma novejma klukama, not *s těmi novými klukama). This is closely tied to the broader Common Czech -ej vowel shift.

You will hear this pattern constantly after prepositions like s/se (with) or mezi (between/among). Note the phonetic rule of preposition vocalization: the preposition s frequently vocalizes to se before these dual forms for ease of pronunciation (e.g., se svýma očima, se třema nohama).