Это красивая безделушка.

Because это here does not directly modify безделушка.

In Это красивая безделушка, это means this is... and stays in the fixed form это.

Compare:

• Это красивая безделушка. = This is a pretty trinket.
• Эта красивая безделушка... = This pretty trinket...

In the second example, эта directly goes with the feminine noun безделушка, so it has to agree with it.

Because it agrees with безделушка, which is:

• feminine
• singular
• nominative

Russian adjectives change their endings to match the noun they describe.

So you get:

• masculine: красивый
• feminine: красивая
• neuter: красивое
• plural: красивые

Since безделушка is feminine singular, красивая is the correct form.

It is in the nominative singular.

This sentence is a simple identification sentence: Это X = This is X. In that pattern, the noun is normally in the nominative:

• Это книга.
• Это дом.
• Это красивая безделушка.

So безделушка is the basic dictionary form.

Russian usually leaves out the present-tense form of to be in sentences like this.

So:

• Это красивая безделушка.

literally works like:

• This [is] a pretty trinket.

The verb быть is usually omitted in the present tense.

But in other tenses it appears:

• Это была красивая безделушка. = This was a pretty trinket.
• Это будет красивая безделушка. = This will be a pretty trinket.

Because Russian has no articles.

English needs a/an and the, but Russian does not. Whether something is definite or indefinite is understood from context.

So безделушка can mean:

• a trinket
• the trinket

depending on the situation.

In this sentence, English naturally uses a.

Безделушка means a small, usually decorative or not very useful object: a trinket, knick-knack, or bauble.

It often suggests something:

• small
• ornamental
• not especially important
• sometimes a little frivolous

The tone can vary:

• neutral: just a small decorative object
• affectionate: a cute little thing
• slightly dismissive: something insignificant

So it is not usually a serious or important item.

The stress is:

• Э́то краси́вая безделу́шка.

A rough English-friendly guide:

• EH-tuh kra-SEE-va-ya byez-dye-LOOSH-ka

A few notes:

• красивая has the stress on си
• безделушка has the stress on лу
• the ending -ая in красивая is often pronounced smoothly, not as two fully separate strong vowels

They are similar, but not the same.

Это красивая безделушка means:

• This is a pretty trinket.
• You are identifying what something is.

Эта безделушка красивая means:

• This trinket is pretty.
• You already know which trinket you are talking about, and now you are describing it.

So:

• это = this is...
• эта безделушка = this trinket

That distinction is very important.

Yes, Russian word order is flexible, but the neutral, natural order here is:

• Это красивая безделушка.

That is the standard way to say it.

If you change the order, the emphasis can change too, and some versions may sound marked or context-dependent. For a learner, it is best to keep this neutral order unless you have a reason to emphasize something specific.

So this sentence is a good basic model:

• это
  • adjective + noun

It can cover both, depending on context.

Красивый / красивая / красивое is a general word for something visually attractive. In English, that may come out as:

• beautiful
• pretty
• lovely
• nice-looking

With безделушка, English often chooses pretty or beautiful, depending on tone. In everyday speech, pretty trinket is often the most natural choice.