Эта вещь оказалась подделкой.

Because вещь is a feminine singular noun in the nominative case, and эта has to agree with it.

Agreement here is:

• эта = feminine singular nominative
• вещь = feminine singular nominative

So:

• этот would be masculine
• это would be neuter

That is a very common point of confusion.

In Russian, nouns ending in -ь can be either masculine or feminine. The ending alone does not tell you the gender with certainty, so you often just have to learn it together with the word.

вещь is feminine, and you can see that from agreement:

• эта вещь
• вещь оказалась

Both эта and оказалась show feminine agreement.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is эта вещь, which is feminine singular, so the verb is feminine singular too:

• masculine: оказался
• feminine: оказалась
• neuter: оказалось
• plural: оказались

So оказалась means that the thing turned out to be something.

Not exactly.

оказаться usually means to turn out to be, to prove to be, or to be found to be. It suggests that the truth became clear later.

So this sentence is not just saying the thing was a fake. It is saying that it turned out to be a fake.

Compare:

• Эта вещь была подделкой. = This thing was a fake.
• Эта вещь оказалась подделкой. = This thing turned out to be a fake.

The second version carries a sense of discovery or surprise.

Because after verbs like быть, стать, оказаться, and some similar verbs, the noun that describes what something is / became / turned out to be is often put in the instrumental case.

The dictionary form is:

• подделка = fake, forgery, counterfeit

Its instrumental singular form is:

• подделкой

So:

• Эта вещь оказалась подделкой.

This is a very standard pattern in Russian.

It is the instrumental singular of подделка.

Formation:

• nominative: подделка
• instrumental singular: подделкой

For many feminine nouns ending in -а, the instrumental singular ends in -ой or -ей. Here it is -ой, written as подделкой.

They are similar, but the nuance can differ a bit.

• подделка = a fake, forgery, counterfeit; a very common neutral word
• фальшивка = fake, forgery; often a bit more colloquial
• поддельная вещь = a fake item, literally a counterfeit thing/item

In your sentence, подделка works very naturally because it names what the object turned out to be.

Russian uses aspect, and here the perfective form оказалась fits because the sentence refers to a completed discovery: at some point, it became clear that the item was fake.

• оказалась = turned out, it became known
• оказывалась = was turning out / used to turn out / repeatedly turned out, depending on context

In this sentence, the speaker is talking about one specific result, so оказалась is the normal choice.

Russian word order is flexible, but the given order is the most neutral and natural:

• Эта вещь оказалась подделкой.

Other orders are possible, but they change the emphasis:

• Подделкой оказалась эта вещь.
  Emphasis on it was this thing that turned out to be a fake.

• Эта вещь подделкой оказалась.
  More conversational or stylistically marked.

For a learner, the original sentence is the best default pattern.

The main stresses are:

• Э́та
• вещь
• оказа́лась
• подде́лкой

So roughly:

Э́та вещь оказа́лась подде́лкой.

A learner should especially remember the stress in:

• оказа́лась
• подде́лка
• подде́лкой

Stress in Russian is important because it is not always predictable.