Some Japanese grammar questions?

Question:

I'm really curious about some details of Japanese grammar.

(1) What is the use of "koto"?
-The only use I learned was that it is used to establish something that has been done/experienced. example, "tabeta koto ga arimasu".

But lately I've been listening to Japanese records and I notice people use it in other ways.

(2) When to combine two particles such as "に　は”　”で　は”？

Answer:

Koto means "thing", or turns something into a "thing" (i.e. a noun).

So, like what you know...

Tabeta koto ga arimasu. (I've eaten that before.)

LIterally, I have the thing (experience of) of having eaten that.

You can think of it like "mono" means "thing" for physical things, whereas "koto" means "thing" for mental/non-physical things.

Sore ha ii mono. That's a good thing (physical object.)

Sore ha ii koto. That's a good thing (that quality, that personal characteristic, that way of thinking, etc.)

As far as combining two particles goes, the "ha" just puts emphasis on whatever you're talking about.

Nihon ni sunderu. I live in Japan.

Nihon ni ha, takusan ii hito ga iru. In Japan (emphasizing Japan as opposed to other places), there are lots of good people.

(Note, you wouldn't say, "Nihon ni ha sunderu." You would usually want to say more than just one word after "... ni/de ha")