As a 接尾語（せつびご） (suffix), 中（ちゅう）
is frequently used to highlight a period of time (or space). By attaching the 格助詞（かくじょし） (case marking particle) に
, the sentence can be modified, to show that (B) is happening/existing throughout the timeframe/space of (A). This is often translated as ‘throughout (A), (B)’, or ‘during (A), (B)’. First, let’s take a look at 中（ちゅう）
being used to show events that are in process.
I am in the middle of working right now.
The deadline is sometime this month.
Now, to show that a (B) event is happening within (A), simply add the 格助詞（かくじょし） に
to show a ‘temporal location’. This will be done by attaching に
to the end of any noun that you would like to indicate as the being the ‘space’ within which another thing exists/occurs.
He reads books during his break.
I don't like being talked to while studying.
Please submit your papers by today.
I want to go to Italy this year.
In these examples, we can see that 中（ちゅう）に
is sometimes read/said as 中（じゅう）に. The basic rule for when to use ちゅう or when to use じゅう is as follows:
- Expresses a very specific (usually small) amount of time/space, or something that is not always the same (an amount of time/space that can change, depending on the person/situation). This ‘thing’ that ちゅう highlights is usually a ‘process’, and therefore means that (A) is ‘in the process’ of happening.
Examples - A meeting, class, visit to somewhere.
- Expresses a predetermined amount of time/space that will always remain the same, regardless of the person/situation. Therefore, じゅう does not highlight a ‘process’, but focuses on the ‘length’ or ‘space’ itself. This tends to mean ‘all throughout’, as it focuses on the full amount of time/space.
Examples - A day, year, house, town or country.
There is so much garbage throughout my house, that there is no room to walk.
There is also a difference between 音読（おんよ）み (chinese readings of words), and 訓読（くんよ）み (japanese readings of words).
じゅう is used more frequently with 訓読（くんよ）み - (今年中（ことしじゅう）, 夏休（なつやす）み中（じゅう）, 今晩中（こんばんじゅう）)
ちゅう is used more frequently with 音読（おんよ）み - (水中（すいちゅう）, 会議中（かいぎちゅう）, 試合中（しあいちゅう）.