First off, a small correction: it has nothing to do with the orthography (I have retagged the question); the sounds alternate, and the written letters only follow the pronunciation.
The Consonant Alternation is a common phenomenon in Slavonic languages.
Particularly, Ukrainian has numerous cases of Alternation.
/t/ ←→ /t͡ʃ/ alternation has been caused by Iotation in Proto-Slavonic language and occurs in verbs having
-ти- at the end of the stem (hence the iotation
ti → tji).
- to want — хотіти — хочу (1SG), хоче (3SG), хочуть (3PL);
- to fly — летіти — лечу (1SG), however летить (3SG), летять (3PL);
- to pay — платити — плачу́ (1SG);
- to dam up — гатити — гачу́ (1SG);
- to spin — крутити, вертіти — кручу, верчу (1SG);
However, the case of "хоча" is even more interesting. According to Vasmer's dictionary, хоча is a remnant of Optative1 mood of the verb хотіти, and the Optative has been usually expressed with the suffixes
iê, iiê in Proto-Indo-European (PIE) and Proto-Slavonic (PSl) — mind the iotation again.
So, in Ukrainian the form
-tiê has evolved into
-t͡ʃa, while in russian it reduced to
Note that the russian retains
-ч- in this very verb when used in 1st person singular:
- to want — хотеть — хочу (1SG).
find a such rule helping me to identify words more quickly.
This must be the most difficult part. The phonology of Slavonic languages have a lot of sound elisions, alternations, reductions, and epenthesis. There are some formal rules that go as deep as to PIE, and many of them have exceptions, too. The
/t/ ←→ /t͡ʃ/ alternation is just one of them.
This may sound unscientific, :-) but if I were you, I would not depend on deeply-buried formal rules here. Just like an English learner might be marveled with the vowel in "women"; Maybe there's a historical reason and a formal rule governing why this word is pronounced this way, but it would be easier to simply remember the word and focus on usage instead.
(1) Optative is a grammatical mood that indicates a speaker's wish or hope. Speaking about its modality, it stands between the Imperative (a very strong wish) and the Subjunctive (a soft desire or hope).