Mysterious Japanese grammar: using the を(wo) particle with だ (da)・です (desu) ?

The Japanese particle を (called “wo” or “o”) is argubly one of the most straightforward particles in the language, with only one major use: describing the direct object of an action. First, let’s look at a simple example of its usage:

• 僕は本を読んでいます (boku wa hon wo yonde imasu)
• I am reading a book.

Here, the を particle is put after 本 (book), which is the direct action of a form of the verb 読む (to read). By the way, as you probably know in English there is no explicit way to specify the direct object of a verb, and the only way to indicate it indirectly is with word order.

However, rules in languages are rarely without exceptions. Here is a sentence that uses を in a way that may surprise you:

• 旅行を準備中です。 (ryokou wo junbichuu desu).
• (I’m) preparing for a trip.

Technically speaking, the only verb in this sentence is です (desu), which as you probably know means “is”/”are” (this is formally called “the copula”) and doesn’t normally use the を particle. What would the direct object of “is” or “are” be anyway? When creating sentences of the form “A=B”, generally you use は (wa) or が (ga), for example:

• 僕は男です。 (boku wa otoko desu)
• I am a man.

Using を anywhere in this sentence would ungrammatical.

So going back to the previous example, the grammar gets a little tricky because of word 準備中. Using 中 (chuu) after a noun means that you are in the process of doing that action (similar to the “-ing” verb form in English). Therefore, 旅行を準備中です effectively means:

• 旅行を準備しています。 (ryokou wo junbi shite imasu)
• (I’m) preparing for a trip.

Here, a form of the verb する (suru) is being used as a helper verb to transform 準備 (which is technically a noun) into a verb. This is done very commonly in Japanese with a variety of nouns.

Now it should be a little clearer why を is used in the phrase 旅行を準備中です. As the 準備中 part is essentially an abbreviation of 準備しています, the を particle simply is retained to specify the direct object of 準備中 (which is acting a verb), not the actual verb, です. The verb 準備 is just an example, you can use many other verbs such as 勉強、作成、etc.

In case you are considering using 中 with verbs, be aware that this has a slight stiff, formal nuance to it, as opposed to using a form of する。One place you will often seen it used in print (or in a computer application) where there may be limited space available.