I wish I could understand Japanese

Uh, sort of. You have the Katakana part correct.

Hiragana is for everything. It's used in everything, but Kanji is used instead in most cases for better understanding. For instance, like English, Japanese has many homophones (words that sound the same but have different meanings). Words like 端 (hashi - edge), 橋 (hashi - bridge), and 箸 (hashi - chopsticks) need Kanji to be differentiated for instructions or rules so they're not misinterpreted. In vocal Japanese, however, that's unavoidable, so it's all about context for spoken JP.

In one Japanese folk tale, there's a kid who reads a sign that says, "do not cross this bridge," but it was written in hiragana, so he took it out of context (out of spite, of course) and said that it meant, "do not cross along the edge of the bridge."

I've also noticed katakana used for emphasis in writing. Kinda like the Japanese version of CAPS LOCK FOR CRUISE CONTROL. Some words I've encountered that aren't loan words are also written in katakana. Like 馬鹿, which is usually written in katakana as バカ instead.