If 「は」 marks a topic, and 「が」 marks a subject, what does 「です」 do in terms of nuance?

Question

When using 「は」 it means the word before it is the topic of the sentence, it also implies contrast. (e.g. 「私は犬です」 means, "As for me, I am a dog" This means that I'm not a cat.)

When using 「が」 it means the word before it is a subject, it is picked out of several possibilities that may also work. (e.g. 「私が犬です」 means, "I am the one who is the dog" This means that I am the dog who is being looked for, or picked out of other creatures who might be dogs, depending on context)

These are the nuances of the particles that I'm aware of (Please post others, as I'd really like to learn as much about them as possible).

「です」 is shortened from 「である」, if it was theoretically natural to say 「ある」 and move the 「で」 to the beginning of the sentence to say a noun is also another noun, (e.g. 「犬で私はある。」 has the exact same meaning as my first example, except it is more polite and usually used in writing) what nuances would 「犬で」 carry?

Answer 1

You technically can split である and place a topic or something in between, but this is really very unusual (with one exception, which I'll mention in a moment). 「犬で私はある」 is technically possible, but is rare enough to be quite noticeable - it places a strong focus on the predicate noun (犬 here), which is further amplified by the rarity of the construction. You can only very rarely place a whole noun in between で and ある, and I would avoid doing it if you're not trying to be fancy - certainly I would not expect to hear that in colloquial speech.

You can, however, quite easily topicalise the argument that's marked with で, resulting in は appearing in there:

私は犬ではある.

This construction generally is used in contrast situations, meaning that on its own, this example doesn't sound quite like a complete sentence. It makes a bit more sense in a context like this:

私は猫ではないが、犬ではある.

which means something like 'I'm not a cat, but I am a dog.'