Vocabulary, in depth
i-adjectives and na-adjectives: why they are kept apart
Japanese adjectives conjugate, and they do it in two ways. The i-adjectives change their ending as if they were verbs: 高い (tall, expensive) becomes 高くない in the negative and 高かった in the past. The na-adjectives are left untouched and take です, じゃない or だった after them: 静かです, 静かじゃない, 静かだった. To go in front of a noun, the first kind go as they are, 高い山, and the second need な: 静かな町. Mixing up the two patterns is one of the most frequent mistakes at the start.
The trap is that the ending is misleading. きれい (pretty, clean) and 嫌い (disliked) end in い and are na-adjectives: きれいな花, きれいじゃない. And the most used adjective of all, いい (good), is irregular: it conjugates from よい, so its negative is よくない and its past, よかった. That is why the adjectives in Hanasuka's vocabulary are not lumped together: i-adjectives and na-adjectives each have their own filter, and can be studied and quizzed separately.
Verbs: the group decides everything
A Japanese verb is learned together with its group, because every form depends on it. Group 1 verbs change the final vowel (話す, 話します, 話さない); group 2 verbs end in -iru or -eru and drop the る (見る, 見ます, 見ない); and group 3 holds する and 来る, which are irregular. Many verbs are formed with する after a noun: 勉強する, to study; 電話する, to phone. In those cases, learning the noun also means learning the verb.
The exceptions are worth knowing early: 帰る (to go back), 入る (to enter), 走る (to run), 知る (to know) and 切る (to cut) end in -iru or -eru and belong to group 1, so they make 帰ります and not «帰ます». That is why every verb in the vocabulary carries its group and its polite form in its note, the polite form being the one used most at the start. And they are studied in the dictionary form, which is the one that appears in any dictionary and the base of the plain forms.
Counters: books are not counted like people
In Japanese, numbers do not go alone: they take a counter that depends on what is being counted. 人 for people, 冊 for books, 枚 for flat things like sheets of paper or shirts, 本 for long thin things like bottles or pencils, 匹 for small animals and 台 for machines and vehicles. And there is a wildcard, the Japanese numbers with つ (ひとつ, ふたつ, みっつ), which works for almost any object up to ten when you do not know which counter applies.
What is hard is not so much the counter as the sound changes. 一本 is read いっぽん, 三本 さんぼん and 六本 ろっぽん; 一人 is ひとり and 二人 ふたり, but 三人 is already さんにん. These changes are learned through use rather than rules, starting with the most frequent counters. Hanasuka's vocabulary puts numbers and counters in a class of their own, with notes like these, and the N5 theory devotes a whole unit to them.
Words that sound the same, and words with several meanings
Japanese has few possible syllables, so words that sound the same and are written differently abound. 鼻 (nose) and 花 (flower) are both はな; 橋 (bridge) and 箸 (chopsticks) are both はし; and 訪ねる (to visit) and 尋ねる (to ask) are both read たずねる. In writing, the kanji clear up the doubt; in speech, context does, and sometimes pitch accent. It is one of the reasons written Japanese cannot do without kanji.
And the other way round: a single Japanese word covers what in English are several. 高い is «tall» and also «expensive»; 聞く is «to hear», «to listen» and «to ask»; 青 is «blue», but also the green of a traffic light. That is why the meaning of each word in the vocabulary gives its main senses separated by commas, and when typing in the quiz any of them is accepted. And that is also why a word that shares a sense with the right one is never used as a decoy: there has to be only one correct answer.