〜ずつ

Number + counter + ずつ (2つずつ, 1人ずつ, 3枚ずつ, 5ページずつ). Also 少し + ずつ (little by little) and the question form 何 + counter + ずつ (何枚ずつ). ずつ comes directly after the amount. The person or thing that receives the amount is often marked with に: 1人に2つずつ (two for each person).

• 毎日、漢字を5つずつ覚えます。

  まいにち、かんじをいつつずつおぼえます。

  I learn five kanji every day.

• 1人ずつ部屋に入ってください。

  ひとりずつへやにはいってください。

  Please come into the room one at a time.

• 日本語が少しずつ上手になりました。

  にほんごがすこしずつじょうずになりました。

  My Japanese has got better little by little.

• 毎日、何ページずつ読みますか。

  まいにち、なんページずつよみますか。

  How many pages do you read each day?

• ペンを1人に1本ずつあげましたから、もうありません。

  ペンをひとりにいっぽんずつあげましたから、もうありません。

  I gave one pen to each person, so there are none left.

• 80円の切手と50円の切手を3枚ずつ買った。

  はちじゅうえんのきってとごじゅうえんのきってをさんまいずつかった。

  I bought three 80-yen stamps and three 50-yen stamps.

• カラオケでみんな1曲ずつ歌った。

  カラオケでみんないっきょくずつうたった。

  At karaoke everyone sang one song each.

• Wrong position: ✗ ずつ2つあげます / ✗ 2つりんごずつあげます. ずつ comes directly after the number + counter: ○ りんごを2つずつあげます. When you count things, keep the counter: ✗ りんごを2ずつ → ○ りんごを2つずつ.
• Using ずつ for 'every day / every week'. English 'each' and Vietnamese 'mỗi' cover both ideas, but in Japanese 'every' + time word is 毎: ✗ 日ずつ勉強します → ○ 毎日勉強します. ずつ follows an amount, not a time word: ○ 毎日2時間ずつ勉強します (I study two hours each day).
• Reading the amount as the total. 3人に2つずつあげました does not mean 'I gave two in total'. Each of the three people got two, so the total is six. If the total is two, do not use ずつ: 3人に2つあげました.

だけ (only)

Both come right after an amount, but the meaning is different. 1つだけ取ってください means 'Please take only one' (one is the limit). 1つずつ取ってください means 'Please take one each' or 'Please take them one at a time' (the same amount for each person or each time).

period + に + 〜回 (... times per period)

に after a period tells how often: 1日に3回薬を飲みます (I take medicine three times a day). ずつ after an amount tells how much each time: 薬を2つずつ飲みます (I take two tablets each time). They are often used together: 1日に3回、2つずつ飲みます (three times a day, two each time).

In N5 grammar questions (choose the word for the blank), ずつ appears next to だけ, ごろ and ぐらい after a number + counter. Look for words such as みんなに, 1人に or 毎日 in the sentence: they show that the same amount is repeated, so ずつ fits. In sentence-building questions, keep the order amount → ずつ → verb (2つ → ずつ → あげました). In listening, instructions such as 1人ずつ, 1枚ずつ取ってください and 少しずつ are common, so check whether the number is for each person or the total.