It should be that each word in japanese, except for (as you said) "borrowed words" like アルバイト (part-time job), has its root written in kanji.
So, the part of the word that carries the actual meaning, should be in kanji, while the "grammar part" (like the tense of the verb) it is usually written in hiragana.
Example: 行く (iku, to go) different from 行きました (ikimashita, past tense, gentle form)
食べる (taberu, to eat - one of my favorite words)
Still, there are also many words in japanese that are usually written completely in hiragana (or at least I've found them always in hiragana until now) and are usually particles, prepositions like に , へ , で, or simple words like りんご (ringo, apple).
hope this helps a little bit ;)