In anime, sometimes a character knocks on their head, giggles a tee hee, winks, and puts their tongue out, or a variation of the sort, to say "oops" when they make a mistake, or to pose cutely for the camera, perhaps doing a peace sign. This is called a tehepero てへぺろ (or "teehee pero" in English), also spelled テヘペロ, or with a star てへぺろ☆.
In Japanese, kaogei 顔芸, "face performance," refers to comedic performance (in real life) using the face, specially making weird facial expressions.
From that, in anime and manga, kaogei can refer to basically any facial expression used to communicate a characters' emotions, but specially the intense, ridiculous, or over the top ones, like those often used when a character is shocked or enraged.
In Japanese, the kateikei 仮定形 is one of the six basic inflectable forms of verbs and adjectives.
The term kateikei is used with modern Japanese. The term izenkei 已然形 is used with classical Japanese. The kateikei evolved from the izenkei. Although they're similar, their functions are different.
For example, in nomeba 飲めば, the nome~ 飲め～ part is the kateikei or the izenkei. In modern Japanese, it's the kateikei, and nomeba means "if [you] drink [it]." In classical Japanese, it's the izenkei, and nomeba means "because [you] drink [it]."
In Japanese, the rentaikei 連体形 is one of the six basic inflectable forms of verbs and adjectives.
For example, in {kirei na} neko 綺麗な猫, "a cat [that] {is pretty}," and {shaberu} neko しゃべる猫, "a cat [that] {talks}," the phrases kirei na 綺麗な and shaberu しゃべる are in the rentaikei.
In Japanese, the ren'youkei 連用形 is one of the six basic inflectable forms of verbs and adjectives.
For example, in yomimasu 読みます, "to read (polite)," and yomitai 読みたい, "want to read," the yomi 読み is the ren'youkei form of the verb yomu 読む, "to read."
In Japanese, the mizenkei 未然形 is one of the six basic inflectable forms of verbs and adjectives. It doesn't mean anything on its own, and acts as an intermediary step to construct more complex forms.
For example, in yomanai 読まない, "doesn't read," and yomareru 読まれる, "to be read," the yoma~ 読ま～ is the mizenkei of yomu 読む, "to read."
In Japanese, a kanou doushi 可能動詞, "potential verb," is a shimo-ichidan verb conjugated from a godan verb consonant stem plus ~eru ～eる, which adds an "able to" meaning to it.
For example: yomeru 読める, "to be able to read," or "can read," is the potential verb variant of yomu 読む, "to read."
It is, in practice, the potential form of godan verbs.
In Japanese, go-juu-on 五十音, "fifty sounds," refers to a way to layout the kana 仮名 characters as a table of five columns (vowels) per ten rows (consonants), totaling fifty table cells. This layout is also used to sort Japanese words.
