LEARNERS REJOICE: NO MORE DUAL NUMBER IN POLISH!

A great boon to leanrers of Polish is the fact that the dual form of nouns and adjectives has long ceased to exist. I think it still is used in the south Slavonic languages. In other words, in older Polish you had three numebrs:

SINGULAR krowa, DUAL krowie and PLURAL krowy. Naturally, the meant 14 more cases in singular and plural to learn. Nowadays only a few vestiges of the dual number remain as in: obiema rękoma, oczyma, uszyma (rarely heard). There is the well-known saying: Mądrej głowie dość dwie słowie (in modern Polish: dwa słowa). Traces of the dual also remain in some peasant dialects as in: Co robita? and Chodźwa, na nas już czas.

Then I suppose Slovene remains the sole surviving Slavic language which retains its dual number, yes? Just wondering -:)

I've been learning Polish for about three years and I've never heard of it having a dual number. How long ago was this?

The change started around 15th century but some remnants of dual number forms survive today and are common, although sometimes occurring only in some grammatical forms:

dwu, dwoma
dwadziescia (20)
dwiescie (200)

Czech (and possibly Slovak too,I'm not sure) still retains the dual form for certain nouns, mainly parts of the body which come in pairs.

For that matter, weird 'plurals' of some body parts that come in twos in Polish also are etymologically dual.

eye - oko - oczy (instead of oka)
ear - ucho - uszy (instead of ucha)
hand/arm - ręka - ręce (instead of ręki - I'm not sure if 'w ręku' is etymologically dual or not)

Also there are some optional irregular forms that come from old dual forms like oczyma (alongside regular oczami).

Sing.------------------Dual (only arch.)----Plur. (arch./moderne)

N. (jedna) ręka--------(dwie) ręce---------(trzy, cztery) ręki / ręce
G. (jednej) ręki--------(dwu) ręku----------(trzech, czterech) rąk
D. (jednej) ręce-------(dwu) rękoma-------(trzem, czterem) rękom
Acc. (jedną) rękę------(dwie) ręce---------(trzy, cztery) ręki /ręce
I. (jedną) ręką---------(dwu) rękoma-------(trzema, czterema) rękami
L. (jednej) ręcę--------(dwu) ręku----------(trzech, czterech) rękach

In bold are forms that are not in use any more. The ancient nominative and accusative cases in plural were replaced by the respective cases of the dual. For both the instrumental and locative cases, the dual and plural forms are still in use. We say "mam dziecko na rękach or na ręku". We never say "mam cię w rękach", but "mam cię w ręku" which is an idiomatic expression telling "I am able to control you" in which expression the form "w ręku" fully retains the sense of the archaic dual number: "mam cię w obu (dwu) ręku" (one has only two hands).

The Czech forms are virtually identical. just spelt differently.

oko - oèi - oèima
ucho - uši - ušima
ruka - ruce - rukama

For that matter, I that the old dual -ma forms had replaced the literary plural forms in speech (if not writing). (I might be wrong, I love listening to Czech but the dichotomy between literary and spoken Czech confuses the hell out of me).

Yes, that's right. One of the frustrating things about Czech is that you spend an inordinate amount of time learning the declension of nouns and adjectives (which are just as complex as in Polish) only to discover that in colloquial speech many of these endings are different. You have to at least be able to recognise these alternate endings. For example:

literary Cz instrumental plural syny (masc noun sons) ženami (f women) moři (nt seas)
collo Cz instrumental plural synama (masc noun sons) ženama (f women) mořema(nt seas)

oko, pl. oka: drops of fat floating on the surface of a broth or a sauce
ucho, pl. ucha: handles (of a cup, for example)

Vestiges of a form of dual remain in Irish I believe.