Japanese counts with a suffix chosen by the shape and kind of the thing counted. This page also covers durations and how often something happens.
Also known ascountersjapanese counters助数詞josuushihow long duration
Japanese counts with counters: 〜つ (general things), 〜人 (people), 〜枚, 〜台…
りんごを よっつ かいます。
I'll buy four apples.
きょうしつに がくせいが ごにん います。
There are five students in the classroom.
A number alone is not enough — Japanese adds a counter that fits the kind of thing. The counting expression usually sits right before the verb. The native series ひとつ・ふたつ・みっつ… (〜つ) counts general objects up to ten.
In the textbooksMinna I, lesson 11Genki I, lesson 5
A length of time before the verb says how long: １時間 勉強します.
まいにち いちじかん にほんごを べんきょうします。
I study Japanese for one hour every day.
A duration (１時間 = one hour, ３日 = three days) is placed before the verb with no particle. Don't confuse 時 (a point in time) with 時間 (a length of time).
In the textbooksMinna I, lesson 11
“X times per period”: １週間に ２回 = “twice a week”.
いっしゅうかんに にかい テニスを します。
I play tennis twice a week.
The period takes に and the count uses the counter 回 (times): ［period］に ［number］回.
In the textbooksMinna I, lesson 11