Well, you just have to memorize it. In case of подруга it's in fact will be подруги.
These are the most common Ukrainian consonant alternations:
- г — з — ж нога — нозі — ніжка (leg nom. - leg dat. - leg diminutive)
- к — ц — ч рука — руці — ручка (hand nom. - hand dat. - hand diminutive)
- х — с — ш вухо — у вусі — вушко (ear nom. - ear loc. - ear diminutive)
That's a common joke :D
-Дай мені ручку! (Give me a pen)
-На! (Here you are!) gives a hand
Is it maybe a matter of a sound change called assibilation? In Croatian, for example, assibilation (croat.: sibilarizacija) happens when k/g/h stand before "e" or "i", so they become c/z/s, respectively, with some exceptions (Croatian "c" has that 'ts' sound).
They like the і instead of only using the и like they use ї and ' for verbs.
So when we add "і" to the end of the word it makes it plural? Sorry If I understood that wrong
There are 3 possible endings for plural: и, і and ї.
- ї after an apostrophe or a vowel (сім’ї - families)
- и after a hard consonant (except some words that end in ар, ир): слон - слони (elephants), ручка - ручки (pens)
- і after nouns that end in a soft or a sibilant consonant: тінь - тіні (shadows), миша - миші (mice), вівчар - вівчарі (sheperds)
More about hard, soft and mixed groups here: https://www.duolingo.com/comment/8868469
From the given sentence one can not make an assumption about the gender of the friends, could be only males or only females, right?
In Ukrainian, there are various words you can use for the word "friend". (They are gender-specific.)
'друг', 'подруг', 'приятель' are when one is speaking about a 'male friend'. ('Товариш' is from the Soviet times.)
'подруга', 'приятелька', ('товаришка' is also from the Soviet times.) I have also heard "коліжанка" used to refer to a 'female friend. But I think it may be a regional slang. Although it does sound nice!