Барахолка открыта сегодня.

Because барахолка is a feminine singular noun, and открыта agrees with it.

• открыт = masculine
• открыта = feminine
• открыто = neuter
• открыты = plural

Since барахолка ends in -а, it is typically feminine, so открыта is the form you need.

Открыта is the short form, which is commonly used after a noun to mean is open.

So:

• Барахолка открыта. = The flea market is open.
• Открытая барахолка would mean something more like an open flea market, where открытая is a full adjective modifying the noun directly.

In this sentence, Russian uses the short form because it is making a statement about the condition/state of the flea market.

Not exactly. In this sentence, открыта functions like a short-form adjective/participle meaning open.

English says:

• The flea market is open.

Russian often expresses this without a separate verb in the present tense:

• Барахолка открыта.

So even though English uses is, Russian just uses the noun plus the short form.

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

So English:

• The flea market is open today.

Russian:

• Барахолка открыта сегодня.

The idea of is is understood automatically. This is completely normal and one of the most basic patterns in Russian.

A very common clue is the ending -а.

Most Russian nouns ending in -а or -я are feminine, so a learner will usually assume барахолка is feminine unless there is a special reason not to.

That is why the predicate is feminine too:

• барахолка → feminine
• открыта → feminine

Yes. Russian word order is more flexible than English.

These are all natural:

• Барахолка открыта сегодня.
• Сегодня барахолка открыта.
• Барахолка сегодня открыта.

The basic meaning stays the same, but the emphasis can shift slightly:

• Сегодня барахолка открыта puts more focus on today
• Барахолка открыта сегодня sounds like a neutral statement, or can contrast with another day

Russian word order often reflects emphasis and information flow more than strict grammar rules.

The stress is:

• барахОлка
• открытА
• сегОдня

A rough pronunciation guide:

• барахОлка → buh-ruh-HOL-kuh
• открытА → ut-krih-TA
• сегОдня → see-VOD-nya

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

Барахолка usually means a flea market, junk market, or a place where second-hand or miscellaneous old items are sold.

It is a fairly everyday word and can sound a bit informal. Depending on context, it may suggest a market with used goods, odds and ends, or cheap items.

So it is not just any market; it has a more specific flavor than рынок.

Yes. This is a very common Russian pattern:

[noun] + short-form adjective

Examples:

• Магазин открыт. = The shop is open.
• Библиотека открыта. = The library is open.
• Дверь открыта. = The door is open.
• Окно открыто. = The window is open.
• Магазины открыты. = The shops are open.

Notice how the ending changes to match the noun’s gender and number.

No, not in normal usage.

Барахолка открыта сегодня means The flea market is open today.

If you wanted to say was opened today in the sense of an action, Russian would usually use a different structure, for example something with a past-tense verb or a clearer passive construction.

Here, открыта describes a state: the market is in the condition of being open.