間・
Both pick out a stretch of time. With 間, something goes on for the whole of it. With 間に, something happens at some point before it ends.
the whole time
The students took notes throughout the teacher's explanation.
at some point
While the teacher was explaining, a student asked one question.
Noun
Noun + の + 間（に）
夏休みの間, 会議の間, 留守の間に.
Verb
Verb (usually ている) + 間（に）
待っている間, 寝ている間に. いる goes in as it is, and so does いない: 日本にいる間に, 私がいない間に.
Adjective
い-adjective + 間（に）
子どもが小さい間. A な-adjective takes な: 暇な間に.
The whole time, or a point in it
English "while" and "during" cover both. In Japanese the に decides. It is the に of 三時に: it marks a point in time, here a point somewhere in the stretch.
Throughout summer vacation, I helped at my grandfather's shop every day.
All through the vacation, day after day.
I finished my independent research project during summer vacation.
Done at some point before the vacation ended.
After 間: something that lasts or repeats. After 間に: something that happens or gets done.
I kept looking out the window throughout the class.
Not said: 授業の
(I kept looking out the window throughout the class.)
It went on the whole time, so no に.
What gets done with 間に doesn't have to be quick. It only has to fit inside the stretch.
I read this entire document during the lunch break.
The parcel will arrive between two and four.
With two times it is "between".
While something is going on: ている間
A verb before 間 names something that lasts, so it is usually in ている. It mostly stays ている in a sentence about the past, where English says "was".
The phone rang while I was taking a shower.
While I was living in Japan, I went to Kyoto three times.
While my mother was on the phone, the cat ate the cake.
Each half can be about someone different: my mother, the cat. ながら can't do that. It is one person doing both things.
Four to learn whole
長い間for a long time
Thank you for everything you have done for me all this time.
Said when you leave a job or a host family.
この間the other day
I ran into Mr. Tanaka at the station the other day.
その間にin the meantime
I'll boil the water, so please cut the vegetables in the meantime.
いつの間にかbefore I knew it
Before I knew it, it had gotten dark outside.
Here it is read ま, not あいだ.
Close neighbours
ときN5
Plain "when". ているとき and ている間に often come to the same thing. とき just names the time; 間 is a stretch with a start and an end.
The phone rang while I was cooking.
中N4
Goes straight onto a noun for an activity, with no の: 会議中, 食事中. Common at work and on signs.
There was an earthquake during class.
までにN4
The same に. 五時まで goes on until five; 五時までに is at some point up to five.
I will reply by tomorrow morning.
Check yourself
1Which one fits?
映画の
Choose an answer to see why.
2When does the report get written?
春休みの
Choose an answer to see why.
3Which sentence is natural?
Choose an answer to see why.
More detail
間は
With は it is "as long as": this stretch, set against other times.
As long as the children are asleep, I don't turn the TV on.
間に合う
間に合う, "to be in time", is a word of its own, read ま.
If we hurry, we can still make it.
More sentences
Please keep your cell phone turned off during the meeting.
While I was away, my neighbor took care of my cat.
While the children were young, we often went to the park as a family.
I got dinner ready while the children were asleep.
While waiting for the train, I wrote three emails.
A delivery arrived while I was looking after the house.
While the soup was simmering, I finished making the salad.
I got my driver's license during winter vacation.