[Sashimister]

Originally Posted by ByTheWay

I'm having some difficulty understanding the difference between these two sentences:

MMM has explained the difference in meaning.

ミルズさんはらいしゅうしゅちょうにいきます。 しゅちょう　＞　しゅっちょう

And

ミルズさんがしゅちょうに行くのはらいしゅうです。

　　が, not は

The は＞が mistake is the biggest mistake from a native speaker's viewpoint. You generally cannot have two は's in this kind of a short sentence.

People go on business trips at different times. The one that Mills is going on is next week.

ミルズさんがしゅっちょうにいくの is technically the same thing as a relative clause. The subject particle used inside a relative clause is always が, and never は.

Apart from the textbook stating that ...のは　is identfying everything before it as the topic of the sentence I'm in the dark.

Think of the second sentence this way. By adding のは to a phrase ending in a verb, you are nominalizing that phrase or in my own words, you're forming a relative clause.

ミルズさんは出張に行く (Mills goes on business trips.) is a sentence by itself.

To nominalize it, you add の to its end. Furthermore, to use the nominalized part as the subject of a sentence, you also must use は.

Now you have ミルズさんは出張に行くのは.

However, as I stated above, the subject inside the nominalization (or relative clause) MUST be が. Therefore:

ミルズさんは出張に行くのは becomes ミルズさんが出張に行くのは.

If you are the kind to rely on translation for understanding, which I personally am not, ミルズさんが出張に行くのは can be translated as:

1. the fact that Mills is going on a business trip
2. as for Mills' going on a business trip

らいしゅうです = "it is next week". Since this is about a time, you can change the "fact" part of the translation above to the "time".