El arnés es nuevo.

Because arnés is a masculine noun in Spanish, so it takes el.

A good rule of thumb is that you usually have to learn the gender of each noun as part of the word itself. In this case, it is:

• el arnés = the harness

So the rest of the sentence also matches that masculine singular noun.

The accent mark shows where the stress goes: ar-NÉS.

Without the written accent, standard Spanish spelling rules would make you stress it differently. The accent tells you clearly that the last syllable is stressed.

So it is pronounced roughly like:

• ar-NESS

The é does not sound different from a normal Spanish e in quality; the accent mainly tells you where the stress falls.

A simple English-friendly approximation is:

• el ar-NESS es NWAY-bo

More carefully:

• el = like el
• arnés = ar-NÉS
• es = ess
• nuevo = NWEH-bo

A few pronunciation notes:

• Spanish r in arnés is a light tap here, not a strong English r.
• v in nuevo sounds more like a soft Spanish b.
• The ue in nuevo is a glide, so it starts with something like nw.

Because Spanish usually uses ser for identification and more basic, defining descriptions, and estar for temporary states, location, or changing conditions.

Here, nuevo means new as a description of the harness, so:

• El arnés es nuevo. = The harness is new.

This is the normal choice.

If you said está nuevo, that can also exist in Spanish, but it often means something more like:

• it looks brand-new
• it is in like-new condition

So:

• es nuevo = it is new
• está nuevo = it is in very good / like-new condition

In Spanish, descriptive adjectives usually come after the noun.

So:

• el arnés nuevo = the new harness

And in a sentence with ser:

• El arnés es nuevo. = The harness is new.

Putting nuevo before the noun is possible in some contexts, but it can change the tone or nuance. For a basic learner sentence, the normal pattern is exactly what you see here.

Because the adjective has to agree with the noun in gender and number.

Since arnés is:

• masculine
• singular

the adjective must also be:

• masculine singular → nuevo

Compare:

• el arnés nuevo = masculine singular
• la silla nueva = feminine singular
• los arneses nuevos = masculine plural
• las sillas nuevas = feminine plural

Normally, no.

In standard Spanish, a singular countable noun like arnés usually needs an article or another determiner when it is the subject:

• El arnés es nuevo. = correct

Without el, it sounds incomplete in normal conversation:

• Arnés es nuevo. = not natural in this context

You would usually only drop the article in very special styles, such as headings, labels, or very unusual phrasing.

Yes, but the basic order here is the most neutral and common:

• El arnés es nuevo.

Spanish can sometimes move words around for emphasis, for example:

• Nuevo es el arnés.

But that sounds marked or literary, not like the normal everyday way to say it.

So for learners, the best default is:

• subject + es + adjective

Yes. Arnés is the standard word for harness in many contexts across the Spanish-speaking world, including Latin America.

It can refer to things like:

• a safety harness
• a climbing harness
• a dog harness
• other types of harnesses, depending on context

So it is a normal, useful vocabulary word.

The plural is:

• arneses

So:

• El arnés es nuevo. = singular
• Los arneses son nuevos. = plural

Notice two things change:

• el → los
• es → son
• nuevo → nuevos

Very simply:

Question

• ¿El arnés es nuevo? = Is the harness new?

Negative

• El arnés no es nuevo. = The harness is not new.

Spanish often keeps the same word order and changes meaning with:

• question marks and intonation for questions
• no before the verb for negation