As a
suffix,
中(ちゅう) is frequently used to highlight a period of time (or space). By attaching the
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.
今(いま)は仕事(しごと)中(ちゅう)なんだ。
I am in the middle of working right now.
締(し)め切(き)りは今月(こんげつ)中(ちゅう)なんです。
The deadline is sometime this month.
Now, to show that a (B) event is happening within (A), simply add the
case marking particle 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.
彼(かれ)は休憩(きゅうけい)中(ちゅう)に本(ほん)を読(よ)む。
He reads books during his break. (At some point during the act of taking a break)
私(わたし)は勉強(べんきょう)中(ちゅう)に話(はな)されるのが嫌(きら)いだ。
I don't like being talked to while studying. (At some point during the act of study)
今日(きょう)中(じゅう)にレポートを提出(ていしゅつ)してください。
Please submit your papers by today. (Within today as the limit)
今年(ことし)中(じゅう)にイタリアに行(い)きたい。
I want to go to Italy this year. (Within this year as the limit)
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:
中(ちゅう)に - 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.
中(じゅう)に - 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.
ゴミが家(いえ)中(じゅう)に広(ひろ)がっていて、歩(ある)くスペースがない。
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).