Is there a rule for deciding if we should write こんばんは or 今晩は?


I always have problems deciding if I must write a word in hiragana or in
kanji.

For example, should I write こんばんは or 今晩は ?

I know children would rather write in hiragana but what about for adults ?

Is there any rule (of thumb) I can follow to decide it or is it just left to
the writer's mood ?

Thanks in advance for your time,

Gilles




> I know children would rather write in hiragana but what about for adults?

If 100% of your text consist of kana only without any kanji, that is 100%
weird. Kanjis are used not only to help identify what meaning for for sound,
but also kanjis work as visual dividers for text into small portions for
better eye-brain parsing of text, because there is no spaces in Japanese text.

> Is there any rule (of thumb) I can follow to decide it or is it just left to
> the writer's mood ?

There are cases, which are managed by some rule of thumb.

For example, here is a rule of thumb for two-rooted-verbs.

> 1) If the verb consists of two verbs and the first one is in -te form, the
> second verb is written in hiragana.  
>  For example,  
>  読んでみる [yon-de miru] not 読んで見る.
>
> 2) If the verb consists of two verbs and the first one is in -masu form, the
> second verb is written in kanji for its root and hiragana for its
> conjugation.  
>  For example,  
>  降り出す [fu-ri-da-su] not 降りだす and not ふりだす.

The real mastering of when to use kanji and when to use kana is a consequence
of feeling it. After a year of dealing with Japanese language on daily basis,
such feeling grows itself and there is no desire for any rules anymore.

> For example, should I write こんばんは or 今晩は ?

To make a quick check:  
go to Jisho.org.

For any word or phrase there is a mark "Usually written using kana alone".
Using Jisho.org to check for [kanji or kana] is the "rule of thumb" for
Japanese learners. But the final goal is to skip using Jisho.org and just feel
it.

To make deeper checking:  
go to tatoeba.org.

1) Type "今晩は" (use quotes to find exact match) to see examples of using with
kanji and what phrases are used to translate this form (in your case it is
[tonight], [this evening]).  
2) Type "こんばんは" (use quotes to find exact match) to see examples of using with
kana and what phrases are used to translate this form (in your case it is
[Good evening!]).  
3) Type "good evening", "tonight", "this evening" (use quotes to find exact
match) to see examples of translation: kana or kanji (in your case: check if
there is some usage of kana for [tonight], [this evening] and if there is some
usage of kanji for [Good evening]).

Result of this deep checking for your case is:  
1) They use kana to say [Good evening!]  
2) They use kanji to say [tonight] or [this evening].

