Hiragana, katakana, and romaji can be used interchangably - they're converted into the right format behind the scenes. Kanji can be used on the conjugation and nouns pages - it can't be used to answer questions on the kanji page, for obvious reasons...
Hiragana/katakana conversion works as you'd expect - characters from one kana are replaced by the equivalent in the other. However, the rules for romaji to kana conversion may require some explaining:
Lower-case characters become hiragana; UPPER-CASE characters become katakana. You can see how the conversion process works using the buttons below:
|Hiragana to Katakana Katakana to Hiragana Kana to Romaji Romaji to Kana|