Grammar JLPT Level N3 - Lesson 1

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 (during) 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.

• 食事(しょくじ)中(ちゅう)に電話(でんわ)がかかってくると、イラっとする。
  It ticks me off when I get a phone call while I'm eating.

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.