Is there a way to tell which sentences or structures particle wa can replace particle wo for emphasis in?

Question

I've been told that in the sentence,

kare wa tsugou ga warui koto wa kakusu

wo can be used instead of wa in the last particle like,

kare wa tsugou ga warui koto wo kakusu

but that wa is used instead of wo for emphasis. I ask because may be this is one of those simplified explanations that hides a much more complex topic like we see many times in Japanese.

Is there any hard to grasp rule to use wa instead of wo for emphasis, or can you use wa instead of wo for emphasis in any sentence?

Answer 1

Short answer would be that you can use ha instead of wo in most cases.

One thing you need to be aware of is when Noun + ha can be the subject of the verb.

For example, consider

• Hon wo yomu = (I) read books.

You can use ha (Hon ha yomu) to emphasize your reading books (implying not reading mangas for example). Here hon cannot be the subject of yomu, whence no ambiguity.

Next consider

• Kare no otouto wo mita = (I) saw his brother

Just using ha here (Kare no otouto ha mita) would mean His brother saw (something), implying he didn't. So ha is understood as the subject/topic marker in the standalone sentence.

On the other hand, in a conversation like

• A-san minakatta?
• Kare no otouto ha mita

Here ha is understood as the emphasis/constrast of wo. Technically Kare no otouto ha mita is ambiguous because kare no otouto can be subject or object for mita, but practically it is always disambiguated by contexts.

The kare no otouto example suggests: you can most probably use ha instead of wo when you are responding to someone, but it is not always possible to use ha when you start describing something. That is, if you want to start a conversation about 'you saw his brother', using wo is necessary.

And of course, there may well be many other cases where care needs to be taken. So the other (boring) short answer would be, it depends on contexts.