Moneda este ruginită.

Because -a at the end is the definite article in Romanian.

• monedă = a coin
• moneda = the coin

Unlike English, Romanian usually puts the at the end of the noun instead of using a separate word.

So:

• monedă → indefinite
• moneda → definite

Este means is.

It is the 3rd person singular present tense of a fi = to be.

So in this sentence:

• Moneda = the coin
• este = is
• ruginită = rusty / rusted

Romanian uses este here just like English uses is in The coin is rusty.

Because Romanian adjectives usually agree with the noun in gender and number.

Monedă is a feminine singular noun, so the adjective must also be feminine singular:

• masculine singular: ruginit
• feminine singular: ruginită
• masculine plural: ruginiți
• feminine plural: ruginite

So:

• un ban ruginit = a rusty coin/money piece, masculine noun
• o monedă ruginită = a rusty coin, feminine noun

Yes. Monedă is feminine.

That is why you get forms like:

• o monedă = a coin
• moneda = the coin
• o monedă ruginită = a rusty coin

Because it is feminine, any adjective describing it must match that feminine form.

A rough pronunciation is:

mo-NE-da YES-te ru-ji-NEE-tuh

A few useful notes:

• moneda → stress on ne
• este often sounds like YES-te
• in ruginită, the gi sounds like the j in job
• ă is a short neutral vowel, similar to the a in sofa

So ruginită is approximately ru-ji-NEE-tuh.

Yes, but it means something slightly different in structure.

• Moneda este ruginită = The coin is rusty
  This is a full sentence.

• moneda ruginită = the rusty coin
  This is a noun phrase, not a full sentence.

So the version with este gives you a complete statement.
The version without este just describes the noun.

Usually, Romanian adjectives come after the noun.

So the normal order is:

• moneda ruginită = the rusty coin

Putting the adjective before the noun is sometimes possible, but it is less basic and can sound stylistic, emphatic, or literary depending on the context. For a learner, the safest default is:

• noun first
• adjective after

And in a sentence with to be, you use:

• Moneda este ruginită

In everyday Romanian, ruginit(ă) is commonly used for metal objects and is easy to understand.

Strictly speaking, in science, rust is associated especially with iron, and many coins do not literally rust in that exact way. In more precise language, someone might say oxidată = oxidized.

But as a normal learner sentence, Moneda este ruginită is perfectly understandable and natural enough.

In normal standard Romanian, no. You should keep este in a sentence like this.

So:

• Moneda este ruginită = correct full sentence

If you remove este:

• moneda ruginită

that no longer means The coin is rusty as a full sentence; it becomes the rusty coin.

So for beginners, it is best to treat este as necessary here.

Here are the most useful related forms:

• O monedă este ruginită. = A coin is rusty.
• Moneda este ruginită. = The coin is rusty.
• Monedele sunt ruginite. = The coins are rusty.
• Niște monede sunt ruginite. = Some coins are rusty.

Notice the changes:

• este → sunt in the plural
• ruginită → ruginite to match feminine plural nouns

So Romanian changes both the verb and the adjective when you switch to plural.