Без отрицания это предложение звучит странно.

Because the preposition без always takes the genitive.

So:

• без
  • genitive = without ...
• dictionary form: отрицание
• genitive singular: отрицания

Other examples:

• без сахара = without sugar
• без проблем = without problems
• без ошибки = without a mistake

So без отрицания is grammatically the normal pattern.

Here отрицание means negation in a grammatical or logical sense, not just emotional denial.

In Russian, отрицание can mean:

• denial
• refusal
• negation

But in this sentence, it most naturally means something like a negative element in the sentence, for example не or another form of negation.

So the idea is that if you remove the negation, the sentence sounds odd.

Because это предложение is the subject of the sentence.

The structure is:

• Без отрицания = prepositional phrase, setting the condition
• это предложение = subject
• звучит = verb
• странно = adverb/predicative word

Since предложение is the subject, it stays in the nominative case.

Russian uses звучать very much like English to sound.

It can describe literal sound:

• Музыка звучит громко = The music sounds loud

But it also works figuratively with speech, phrases, and sentences:

• Это звучит странно = That sounds strange
• Эта фраза звучит естественно = This phrase sounds natural

So предложение звучит странно is a perfectly normal way to say that a sentence feels odd or unnatural.

Because странно is being used adverbially / predicatively after звучит.

Compare:

• странный = strange, masculine adjective
• странное предложение = a strange sentence

But after verbs like звучать, Russian usually uses the adverbial form:

• звучит странно = sounds strange
• выглядит хорошо = looks good
• пахнет приятно = smells pleasant

So странно describes how it sounds, not directly the noun by adjective agreement.

Yes. Russian word order is flexible.

The sentence could also be:

• Это предложение без отрицания звучит странно.
• Странно звучит это предложение без отрицания.

But the original version:

• Без отрицания это предложение звучит странно.

naturally puts without negation first, which highlights the condition or frame: if there is no negation...

So the word order helps focus attention, even though the grammar stays clear because of the case endings.

Not exactly, but they are close in meaning.

• без отрицания = without negation
• если убрать отрицание = if you remove the negation

The first is more compact and sounds a bit more analytical or grammatical.
The second is more explicit and conversational.

So in context, both can communicate a similar idea, but без отрицания is shorter and more abstract.

In это предложение, это functions like this or sometimes simply this particular sentence.

Russian это is very common and can be slightly broader than English this in actual usage. It often points to something already known from context.

So in a lesson or explanation, это предложение usually means this sentence—the one we are currently talking about.

Yes, предложение can mean:

• sentence in grammar
• proposal / offer in other contexts

You know which meaning is intended from context.

Here the clues are:

• отрицание suggests grammar or sentence structure
• звучит странно also fits well with language discussion

So here предложение clearly means sentence.

Yes, it is natural, especially in a grammatical or explanatory context.

It sounds like something a teacher, editor, or native speaker might say when discussing wording:

• Без отрицания это предложение звучит странно.

It is clear and idiomatic.
A slightly more conversational version might be:

• Без отрицания это предложение звучит как-то странно.

Adding как-то softens the judgment a little, but the original is absolutely normal.

Because без отрицания is just a normal prepositional phrase, not a subordinate clause or parenthetical expression.

Russian does not normally put a comma after a simple fronted phrase like this:

• После урока мы пошли домой.
• Без слов он всё понял.
• Без отрицания это предложение звучит странно.

So no comma is needed.