Both は and が can mark something important in a sentence, but they do different jobs.
With この建物は立派です, the speaker is talking about this building as the topic and then describing it.
If you said この建物が立派です, it would sound more like:
So は is the more neutral, standard choice here.
です makes the sentence polite.
So:
In this sentence, です does not add tense by itself beyond the present/future-style copula use; it mainly gives a polite tone.
The plain form is:
In casual speech, people may also simply say:
That sounds conversational and informal.
建物 is read たてもの.
It means building. Literally, the kanji are related to:
So it is basically a built thing, which is why it means building.
The spaces are usually just for learners.
Normal Japanese writing would be:
Japanese normally does not use spaces between words the way English does. Textbooks and beginner materials often add spaces to make the sentence easier to read.
Then you would say:
That means:
Notice the difference:
So when 立派 comes before the noun, it needs な.
Yes.
These are all part of the same demonstrative pattern:
So you could say:
The grammar stays the same; only the location/relationship changes.