Hiragana and katakana are phonetic characters, so if you really want to, everything can be written either all in hiragana or all in katakana, or combination of hiragana and katakana. On the other hand, Kanji is a symbolic scripts, so they don't always represent pronunciation at all. And school children spend full 9 years of compulsory education to learn approximately 2,000 Kanji. So for Japanese people, knowing a lot of Kanji is a kind of status symbol of your education level as well. But if you are writing a letter to a young child with a lot of Kanji, you will be either mean, showing off, or inconsiderate. But if you are a journalist and writing an article for a business newspaper, and if you use a lot of hiragana and katakana, you will be seen as ignorant. And since Chinese characters are symbols, if you really want to, you can write other languages with kanji, too. Sorry, I don't know Italian, but if you want to write, say, Spanish in Kanji, it will be; A 私í 私e 好a 食er 魚 y 飲er vino赤. to say something like "A mí me gusta comer pescado y beber vino tinto" En la 大学, 勉強o 日本語 e 英語. "En la universidad, estudio japonés e inglés." 帰宅o a 家 a las 5 de la 午後 毎日. "Regreso a casa a las cinco de la tarde todos los días." 食o la 夕食 前es de las 8, 入o un 風呂 en後, 見o la tele y 私e 寝o. "Tomo la cena antes de las ocho, tomo un baño enseguida, veo la tele y me acuesto." Long long time ago, the Japanese didn't know how to read or write. But when Japanese monks went to China to study Buddhism, they learnt how to write in China, brought Chinese characters back to Japan with them and introduced the writing system. However, since the Japanese syntax is so different from that of the Chinese language, the Japanese needed to invent phonetic symbols to represent particles, conjugations, etc.

Hi Andrea I am aslo studying japanese and I find kanji very confusing since I'm native spanish speaker and having different systems for writting one language is very different for me. Like torusan said, most kanji books have two categories of reading onyomi and kunyomi. The tricky part for me has been to know how to pronounce it everytime I see it written. So far I have not seen a rule exactly. Except that you'll use kunyomi when the kanji stands on its own as a word. Kanji is very importand because they give you the "meaning". For example, you can find two words that are pronnounced the same, but have different meanings and you'll know those meanings by a) context and b) the kanji they're written with. As dor the pronunciation part, I'd recommend you not to try to learn rules for reading. For me it never worked out, on the other hand, I know this sounds crazy, specially if you're just begenning your japanese study, but try to read as mmuch as you can, even if you need a dictionary next to you. I'd wish someone had told me this when I began to study. Trying to learn kanji isolated (like most of kanji books teach) is incredibly slow, and relies on your good memory most of the time. Try searching for bilingual books in amazon, or children books that use furigana so you learn the way THAT kanji is pronounced in THAT word. And re-read once in a while. If you don't know the meaning, use a dictionary or a translator. Kanji books are useful because they teach you the stroke order and it makes your handwritten kanji easier to undestrand, and also for some search engines you need to know how many strokes does a kanji have in order to find it. But my advice would be to get familiar with written kanji since the beginning. Kanji books teach you each kanji has a set of different readings and then they send you to read and infere how it's pronounced in each situation but in isolated words. Reading in context has proven to be way faster and useful for me.

You're actually asking questions that require a long dissertation to answer. When you hear a word that is native Japanese, it can be represented in hiragana and kanji. If you hear a borrowed word, it is represented in katakana. The kanji representation would be the stem/root, the conjugated part would be in hiragana. There are kunyomi and onyomi readings because one just replicates the sound (おんよみ) and the other is the Japanese word for the thing (くんよみ). This grammar guide is a pretty good resource: http://www.guidetojapanese.org/grammar_guide.pdf