kono ryouri ha kantan desu ga, oisii desu.

この means this and must come directly before the noun it modifies. So:

• この料理 = this dish / this food

Unlike English, you cannot usually separate この from the noun. You do not say 料理この.

Because この is a determiner, not a standalone noun. It directly attaches to the noun:

• この料理 = this dish

There is no particle needed between them. Compare:

• この料理 = this dish
• これ = this by itself

So if you use この, it must be followed by a noun.

は marks the topic of the sentence. Here, the sentence is talking about this dish as the topic:

• この料理は = as for this dish...

This does not exactly equal the English subject every time. It sets up what the sentence is about, and then the rest comments on it.

簡単 is a na-adjective. When a na-adjective comes before a noun, it uses な:

• 簡単な料理 = an easy dish

But when it comes at the end of a sentence as a predicate, it does not use な:

• 料理は簡単です = The dish is easy

So 簡単なです is incorrect.

Because おいしい is an i-adjective, not a na-adjective.

• おいしい料理 = a delicious dish
• 料理はおいしいです = The dish is delicious

i-adjectives do not use な before nouns. That is one major difference between i-adjectives and na-adjectives.

Here, が means but.

It connects two ideas:

• この料理は簡単です = This dish is easy
• おいしいです = It is delicious

Together:

• この料理は簡単ですが、おいしいです。
• This dish is easy, but delicious.

This が often gives a mild contrast. The sentence suggests something like: Even though it is easy, it is still delicious.

Because there are two polite clauses being connected:

• 簡単です
• おいしいです

In Japanese, it is normal to keep each part polite like this. So the sentence sounds natural and balanced.

You may also see:

• この料理は簡単ですが、おいしいです。

This is also natural. The first です stays before が, and the second です finishes the sentence politely.

Yes. Once the topic この料理は has been introduced, it continues to apply unless something changes.

So after:

• この料理は = As for this dish...

both 簡単です and おいしいです are understood to describe that same dish.

Japanese often leaves out repeated information when it is already clear from context.

No, it is not strictly necessary, but it is common and helpful.

• この料理は簡単ですがおいしいです。
• この料理は簡単ですが、おいしいです。

Both are fine. The comma simply makes the pause clearer and easier to read.

No. Standard Japanese is normally written without spaces:

• この料理は簡単ですが、おいしいです。

Spaces are often added only in beginner materials to make the sentence easier to read.

Yes, it is natural and easy to understand. It has a slightly contrastive feeling:

• It’s easy, but it’s delicious.

That can imply something like:

• You might expect an easy dish to be less impressive, but it actually tastes good.

So the sentence is grammatically correct and also natural in everyday Japanese.

Not in the same position.

でも can also mean but, but it usually starts a new sentence or a new part of the conversation:

• この料理は簡単です。でも、おいしいです。

That means:

• This dish is easy. But it’s delicious.

So:

• が connects clauses inside one sentence
• でも often begins a separate sentence

Both can work, but they are used differently.