Would it be "cit/ies" or "citie/s"? I'm just starting morphology and I got confused about it.
The plural noun cities may be divided into two morphemes:
cit(y) - free morpheme (also known as the root word)
-ies - bound morpheme (also identified as a suffix)
This type of morphological process is inflectional because there has been a grammatical change to the root word: pluralisation (or an increase in number).
Also note that city has an irregular plural form in writing (not speech) as, by convention, if the word ends in a consonant + y then 'y' changes to 'ies'.
Not entirely relevant to your question but interesting nonetheless.
“Cities” is the plural inflected form of “city”. It has two morphemes: city + the noun-pluralizing morpheme, here realized as the suffix /z/. The spelling change of Y to IE is irrelevant to the morphological analysis.
If you really need to split up the written form, I think you could argue for either the division “citi-es” or “citie-s”. I don’t see any reason to think of the letter I as representing part of the second morpheme, so I would not agree with the division “cit-ies”.