"This is an apple."

Translation: Это яблоко.

[salamista]

Is the hyphen necessary?

[hyperion86]

What is the difference exactly between это and этот?

[Berniebud]

"Этот" is for masculine nouns. "Это" can be either for neuter nouns or as a predicate "This is".

"Эта" is for feminine nouns, and "Эти" for plural.

[NoobLucas]

The explanation of what Berniebud said, should you need it:
http://www.russianforeveryone.com/Rufe/Lessons/Course1/Grammar/GramUnit6/GramUnit6_3.htm

[NikolausEn]

The difference between Яблоки and яблоко?

[Kundoo]

Яблоки = apples

[JoshuaKnic]

It's a bit more complicated than that, though you are correct in this case. Яблоки = apples in this case, but in certain forms (like the genetive, I think) яблоки = apple.

[Kundoo]

Nope, the genitive of "яблоко" is "яблока". There's no singular form of "яблоко" that would be "яблоки".

Nominative: яблоко

Genitive: яблока

Dative: яблоку

Accusative: яблоко

Instrumental: яблоком

Prepositional: яблоке

[bnDo12]

What is the difference between It and This?

[rosemaryta4]

don't have Cyrillic script on my computer

[Nico86873]

I declare this phrase "This is an apple." to be a Nominative Case but I didn't match the translation answer "Это — яблоко." I simply typed "Это яблоко" and it was accepted as correct. Paint me bewildered.

[Kundoo]

I'm not sure what you are trying to say, but the dash is optional in this case. In definitions, like this one, it's mostly used for clarity, so that the sentence didn't look like two words randomly thrown together, but it's not absolutely required. Not to mention that Duo mostly ignores punctuation anyway.