Distinguishing 「話し」 する and 五段 usage

Question

Facts

1. The noun はなし can be written 話 or 話し.
2. In colloquial contexts, you can omit 「を」 from 話{はなし}をする, making it 話{はなし}する.
3. There is also a 五段 verb, 話す{はなす}.
4. The continuative (連用形) form of both 話する and 話す can look like 話し.

The problem

From the facts, you can collect 3 words that look exactly the same but have a different meaning (grammatical function). You can probably rule out the noun 話し with the help of context, but you still have either 話し{はなしし} or 話{はな}し.

The question

How can I know when which one is meant? Or does it make such a big difference that I should even care?

Some examples:

• 話しながら…
• 何人に話した？
• 明日お話します。← This must be する because of お-?
• 彼は大声で話した。

Answer 1

In modern standard Japanese, はなし as a noun is always written as 話, without し. This rule is taught at school today and is fairly strict, although you may see exceptions in old documents. As a verb, はなす is written with okurigana.

話 is not a suru-verb. You usually need a direct object marker を when it's used with する.

シンデレラの話【はなし】をする。 (話 is a noun)

はなしして and はなしした can appear in two situations:

1. When はなし is a noun and the following を is omitted because it's a casual sentence.

   ママ、シンデレラの話【はなし】して！ (話 is a noun)

2. As part of a humble expression お + masu-stem + する.

   本日は、世界の童話についてお話【はな】ししていきます。

To distinguish, first note that omission of を after 話 can appear only in fairly casual conversations. And looking at the modifiers will usually tell whether it's used as a noun or a verb. Unless 話 is clearly modified by an adjectival expression in a casual conversational sentence, you can assume 話して/話した is a verb. In your examples, 「話しながら」「何人に話した」「彼は大声で話した」 are theoretically ambiguous, but you can usually assume these はなし are verbs.

• 先生が楽しい話【はなし】してるよ！ (a noun follows after 楽しい)
• 先生は楽しく話【はな】している。 (a verb follows after 楽しく)

Humble おはなしします should be written as お話しします, because 話す is a verb in this construction (cf. お預かりします, お持ちしましょう). That means your third expression is grammatically wrong, although this mistake is found even among native speakers.