彼が学生だったのは知っています。

N5 彼 kare he N5 学生 gakusei student N5 います imasu there is / exist (living things)

N3 彼 hi / kare, kano, ka.no he, that, the N5 学 gaku / mana.bu study, learning, science N5 生 sei, shou / i.kiru, i.kasu, i.keru, u.mareru, uma.reru, u.mare, umare, u.mu, o.u, ha.eru, ha.yasu, ki, nama, nama-, na.ru, na.su, mu.su, -u life, genuine, birth N4 知 chi / shi.ru, shi.raseru know, wisdom, savoir

N5 〜ている te iru Indicates ongoing action, continuous state, repeated habit, or resulting condition depending on context N5 〜ます masu Used at the end of verbs, polite present or future form, makes the sentence more polite N5 〜が ga Marks the grammatical subject, highlights new information, can also show slight contrast in some sentences N5 〜の no Indicates possession, relation between nouns, nominal explanation, or turns a verb into a noun phrase N5 〜は wa Marks the topic of the sentence, indicates what the sentence is about, can contrast with another element, organizes main information