Counters for things and people, duration, frequency with 〜に〜回, and だけ.
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.
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).
“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.
だけ after a noun or a quantity means “only, just”.
やすみは にちようびだけです。
My only day off is Sunday.
ひとりだけ きました。
Only one person came.
だけ limits neutrally — “this much and no more”: 日曜日だけ, 一人だけ. Minna II adds しか + negative, which stresses that it is not enough (lesson 27).