When does "i" indicate palatalization?
In general, for the vast majority of cases, palatalization occurs at the end of the word, marking either the plural of a noun/adjective:.
un pom înalt - doi pomi înalți
un lup mare - doi lupi mari
o floare - două flori
un melc treaz - doi melci treji
o ferigă - două ferigi
or the second person singular conjugation of a verb:
eu sar - tu sari
eu cred - tu crezi
taci! - -- imperative, there's no first person form
să rup - să rupi -- subjunctive
Note that, in both cases, the "i" that marks the palatalization has the effect of the phoneme /i/ - that is it changes any previous "c"|"g" from a /k/|/g/ sound to a /t͡s/|/d͡ʒ/ sound which suffers the palatalization. So a palatalized /k/ or /g/ only occurs in words ending in "chi"/"ghi":
There are also some words that don't fit in the two categories above but still feature palatalization (e.g. the two examples of palatalized /k/ and /g/ are both nouns in their singular form). Words like these can be used in deriving new words and sadly invalidate the end-of-word rule:
câți = câțiva (how many - some)
nici = nicicând (hard to translate out of context - never)
cinci = cincizeci (five - fifty)
I don't know how to express a rule that would dictate that "câțiva" and "nicicând" have two-syllables, you should probably learn them as they are.