は and が in this sentence


i think my understanding of these two is getting better though i have a
question about this sentence:

新しい合金が開発された。医療用や通信用など、用途は広い。

the が i think i get. it is because the new alloy is new information. the
question i have is about the は used. what confuses me is that i recently read
about this kind of construction:

象は鼻が長い。

it was explained as the nose being its own subject rather than a possessive.
that is why the second part confuses me now because if anything i would think
the second sentence would be constructed something like this:

(新しい合金は)用途が広い。

where the new alloy is now the topic as it has just been introduced as new
information in the previous sentence and its uses are the subject much like
the elephant's nose is the subject in that other sentence.

is it a case where once you introduce something, you can kind of skip a step
in the next sentence and talk about a feature of it using は? for instance if
you were to say something like... 象があります。鼻はながい。

?




I just finished reading a book about Japanese grammar and there was a very
enlightening discussion on the difference of は and が. I'm just a native user
of Japanese trying to rediscover the language, so please don't take this
answer as authoritative or formally correct. It's just my current
understanding of the difference. Probably this isn't new to those who have
been studying Japanese as a second-language, but being native I have never
thought about the difference in a structured way.

Both は and が are used for subjects, but the main difference is that は
indicates where the topic of the conversation is, while が simply indicates
what subjects are for each clause that may exist in a more complicated
sentence. That is why a sentence that contains more than one は sounds very
awkward (the topic focus should be on one and only one item at a time), while
a single は can be accompanied with multiple が in a single sentence.

From that view point, it seems the sentence

> 新しい合金が開発された。医療用や通信用など、用途は広い。

is most naturally rephrased as

> 新しく開発された合金の用途は、医療や通信など多岐に渡る。

I used a slightly different expression 多岐に渡る, which in this context means the
same thing. To me, the key here is that the topic of this particular
conversation lies in the wide variety of use, and not just the fact that the
new alloy was developed. That makes sense given that the new development of
alloy is exciting only because it has broad applications.

What makes slightly complicated is that the original phrasing sounds very
natural when broken up into two like given. The development of alloy itself is
a new piece of information, so it should be introduced in that way before
leading to a sentence that conveys a more exiting piece of information which
really is the topic of the conversation.

As for

> 象は鼻が長い。

the topic of the sentence is an elephant itself. Perhaps the conversation
around this sentence is about elephants, and the fact that their nose is long
is an additional piece of information that got into the conversation.

If instead you say

> 鼻は象が長い。

then the topic of the sentence shifts to nose. Perhaps the conversation was
about nose of animals in general, and someone just mentioned the fact that
elephants have a notably long nose compared to other animals.

On the other hand, both

> 象は鼻は長い and 象が鼻が長い。

sound very awkward to me, since the former is ambiguous as to where the topic
lies, and the latter simply does not makes sense. Likely because 象が長い does not
make sense by itself.

