Это полезная штука.

useful

Breakdown of Это полезная штука.

• это: it/this
• штука: the thing; item (colloquial)

Questions & Answers about Это полезная штука.

Why is there no word for is in this sentence?

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.

Why is полезная ending in -ая?

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

Why is it это, not эта, if штука is feminine?