Это полезная штука.

In Russian, the verb to be is usually omitted in the present tense.

So:

• Это полезная штука. = This is a useful thing.

There is no present-tense is written or spoken here.

Compare:

• Это полезная штука. = This is a useful thing.
• Это была полезная штука. = This was a useful thing.
• Это будет полезная штука. = This will be a useful thing.

In the past and future, forms of быть appear, but in the present they normally do not.

Because it agrees with штука, which is a feminine singular noun.

Russian adjectives change their endings to match the noun in:

• gender
• number
• case

Here:

• штука = feminine singular
• so полезный changes to полезная

Basic forms of useful:

• masculine: полезный
• feminine: полезная
• neuter: полезное
• plural: полезные

Examples:

• полезный совет = useful advice / a useful tip
• полезная штука = a useful thing
• полезное приложение = a useful app
• полезные книги = useful books

This is a very common question.

In Это полезная штука, это is not agreeing with штука the way an adjective would. Here это functions more like this is / that is / it is.

So the structure is roughly:

• Это = this/that/it is
• полезная штука = a useful thing

That is why you use это, not эта.

But эта is used when it directly modifies a feminine noun:

• эта штука = this thing

So compare:

• Это полезная штука. = This is a useful thing.
• Эта штука полезная. = This thing is useful.

Both are correct, but they are built differently.

Штука literally means thing, item, or gadget/device, depending on context.

It is a very common everyday word, and it can sound a little informal or conversational. People often use it when they mean:

• a thing
• some object
• some device
• some trick/feature/mechanism in a broader sense

Examples:

• Интересная штука. = An interesting thing.
• Удобная штука. = A handy thing.
• Классная штука. = A cool thing.

It is often used when the speaker is being casual, or when the exact category of the object is not important.

Not exactly, though they often overlap.

• вещь is a more general word for thing
• штука is often more colloquial and can sound more like thingamajig, device, gadget, or just a casual thing

Compare:

• Это полезная вещь. = This is a useful thing.
  Slightly more neutral/general.
• Это полезная штука. = This is a useful thing.
  More conversational, often a bit more vivid.

If you are talking about an object, tool, app, feature, or some practical item, штука sounds very natural.

A helpful stress guide is:

Э́то поле́зная шту́ка.

Approximate pronunciation:

• Это = EH-tuh
• полезная = puh-LYEZ-nuh-ya
• штука = SHTOO-kuh

A more precise pronunciation in IPA is roughly:

[ˈɛtə pɐˈlʲeznəjə ˈʂtukə]

A few tips:

• ш is a hard sh sound
• the stress in полезная is on ле
• the stress in штука is on the first syllable

It is in the nominative case.

This sentence has the structure:

• Это
  • noun phrase in the nominative

So:

• полезная штука is nominative singular feminine

You can tell from the forms:

• полезная = nominative feminine singular
• штука = nominative feminine singular

If the sentence changed and another structure required a different case, the forms would change. For example:

• Нет полезной штуки. = There is no useful thing.
• Я пользуюсь полезной штукой. = I use a useful thing.

But in your sentence, it is nominative.

Yes. Russian word order is flexible, but different orders sound slightly different in emphasis.

The most neutral version is:

• Это полезная штука.

Other possible versions:

• Полезная штука, это.
  Possible, but marked and much less normal in everyday speech.
• Штука полезная.
  Means The thing is useful or It’s a useful thing, with a slightly different feel.
• Эта штука полезная.
  This thing is useful.

For learners, Это полезная штука is the safest and most natural version for this meaning.

Yes, but it means something slightly different.

• Это полезная штука. = This is a useful thing.
• Это полезно. = This is useful.

So:

• полезная штука is a noun phrase: a useful thing
• полезно is a short-form/adverbial predicate meaning useful in a general sense

If you want to describe the object as a thing, use:

• Это полезная штука.

If you just want to say that something is useful, use:

• Это полезно.

It is natural and conversational, mainly because of штука.

The sentence is not slang, and it is not wrong in formal Russian, but it sounds more like everyday speech than careful written style.

• Это полезная штука. = casual, natural
• Это полезная вещь. = a bit more neutral
• Это полезный предмет. = more formal, but often less natural unless you really mean a physical object

So yes, it has a mildly informal, spoken feel.

Yes. In many contexts, это can be translated as:

• this
• that
• it

The exact English choice depends on context.

So Это полезная штука could mean:

• This is a useful thing.
• That is a useful thing.
• It’s a useful thing.

Russian often does not force the same this/that/it distinction that English does in a sentence like this.

You add не before the adjective or predicate phrase:

• Это не полезная штука. = This is not a useful thing.

But in real speech, Russians often prefer:

• Это бесполезная штука. = This is a useless thing.

That sounds more natural if you mean the thing has no use.

You could also say:

• Эта штука не полезная. = This thing isn’t useful.
• Это не очень полезная штука. = This is not a very useful thing.

The dictionary forms are:

• это = this/that/it
• полезный = useful
• штука = thing, item, gadget

Even though the sentence uses полезная, the dictionary form is полезный, because Russian adjectives are usually listed in the masculine singular nominative.

Russian has no articles like a or the.

So Russian simply says:

• полезная штука

Depending on context, English may translate that as:

• a useful thing
• the useful thing
• just useful thing in some special contexts

In this sentence, a useful thing is the most natural English translation, but Russian itself does not explicitly mark that difference.

It can refer to both.

Even though штука often suggests a physical object or gadget, in everyday Russian it can also refer to:

• an idea
• a method
• a feature
• a trick
• a process

For example, someone might say this about:

• an app
• a kitchen tool
• a grammar rule
• a software function
• a life hack

So Это полезная штука is quite flexible and often means something like That’s really useful in a casual way.