sono sara ha marui desu ga, kono bentoubako ha sikakui desu.

You’re right: normal Japanese is usually written without spaces.

So this sentence would normally appear as:

その皿は丸いですが、この弁当箱は四角いです。

The spaces in your example are just a learning aid to help beginners see the parts of the sentence more clearly.

Both are demonstratives that come before a noun.

• この = this ...
• その = that ...

So:

• この弁当箱 = this lunch box
• その皿 = that plate

A useful pattern is:

• この + noun = this ...
• その + noun = that ...
• あの + noun = that ... over there

Also compare:

• これ / それ / あれ = this / that / that over there by themselves
• この / その / あの must be followed by a noun

は marks the topic.

In this sentence, it sets up each item as the thing being talked about:

• その皿は = as for that plate ...
• この弁当箱は = as for this lunch box ...

Here, は also gives a contrastive feeling, especially because it appears in both parts of the sentence. It’s like:

• as for that plate, it’s round, but
• as for this lunch box, it’s square

So this is not just identifying subjects; it’s also comparing the two objects.

Yes. 丸い and 四角い are い-adjectives, and they can already end a sentence in plain style:

• その皿は丸い。
• この弁当箱は四角い。

Adding です makes the sentence polite:

• 丸いです
• 四角いです

This is completely normal in modern Japanese.

A very important point:

• with an い-adjective, you do not say 丸いだ or 四角いだ
• the polite form is 丸いです, not 丸いだです or anything like that

Here, が means but.

So:

• 丸いですが = is round, but ...

It comes after です because the whole first clause is in polite style:

• 丸い = plain
• 丸いです = polite
• 丸いですが = polite + but

Compare:

• その皿は丸いが、... = plain style
• その皿は丸いですが、... = polite style

This が is not the subject marker が.
You can tell because it comes after the predicate and connects two clauses.

You could use けど too.

• が and けど can both mean but
• が sounds a bit more formal or written
• けど is very common in conversation

So these are both natural:

• その皿は丸いですが、この弁当箱は四角いです。
• その皿は丸いけど、この弁当箱は四角いです。

For a textbook sentence, が is very common.

Because here Japanese normally uses the adjective forms:

• 丸い = round
• 四角い = square / boxy / angular

Those are the natural words for describing the shape of an object like a plate or lunch box.

While 丸 and 四角 exist as nouns, saying shape descriptions with 丸い and 四角い is much more straightforward here.

So for everyday object description:

• 皿は丸い = natural
• 弁当箱は四角い = natural

Not necessarily.

In everyday Japanese, 四角い often means something like:

• square
• rectangular
• boxy
• having corners

So it does not always mean a mathematically perfect square.

That’s why 四角い弁当箱 sounds natural even if the lunch box is more rectangular than square. The word is often about the general shape: not round, but cornered.

Yes, it’s basically natural.

The most natural written version is simply:

その皿は丸いですが、この弁当箱は四角いです。

A few notes:

• remove the teaching spaces
• the comma is fine, but not absolutely required
• the level of politeness is consistent

So this is a good, normal beginner-level contrast sentence.

They are pronounced:

• 皿 = さら
• 弁当箱 = べんとうばこ

So the whole sentence is read:

その さら は まるい ですが、この べんとうばこ は しかくい です。

A learner might expect some sound change in 箱, but here it stays ばこ, so 弁当箱 is べんとうばこ.