Los prismáticos están aquí.

Because prismáticos is a masculine plural noun in this meaning. In Spanish, articles have to match the noun’s gender and number:

• el = masculine singular
• la = feminine singular
• los = masculine plural
• las = feminine plural

So los prismáticos is the correct form.

In Spain, prismáticos usually refers to binoculars, and it is normally used in the plural, just like English often treats binoculars as a plural item.

So:

• los prismáticos = the binoculars

Even though it refers to one object, that object has two matching optical parts, so Spanish commonly uses the plural form.

Because the subject, los prismáticos, is plural. In Spanish, the verb must agree with the subject in number.

• el libro está aquí
• los prismáticos están aquí

Since los prismáticos is plural, the verb must also be plural: están.

Because estar is used for location.

This sentence tells you where the binoculars are, so Spanish uses estar:

• Los prismáticos están aquí. = location

A very common rule learners meet is:

• ser = identity, description, origin, time, etc.
• estar = location, condition, state

So when something is here, there, at home, on the table, and so on, you normally use estar.

Aquí is an adverb of place meaning here. It tells you the location of the binoculars.

In the sentence:

• Los prismáticos = the subject
• están = are
• aquí = here

So aquí answers the question where?

Yes. That word order is completely natural in Spanish.

Both are correct:

• Los prismáticos están aquí.
• Aquí están los prismáticos.

The difference is mainly one of focus or emphasis:

• Los prismáticos están aquí focuses a bit more on the binoculars
• Aquí están los prismáticos focuses a bit more on here they are / here is where they are

Not everywhere. In Spain, prismáticos is very common and natural. In many parts of Latin America, binoculares is often more common.

So for Spain:

• los prismáticos sounds very natural

For many Latin American varieties:

• los binoculares may be more common

Both can be understood, but regional preference matters.

Because prismáticos is stressed on the third-to-last syllable: pri-SMÁ-ti-cos.

Words stressed on the third-to-last syllable are called esdrújulas, and in Spanish they always take a written accent.

So the accent mark shows the correct stress:

• prismáticos

Usually yes, if you mean the binoculars in a normal sentence.

Spanish uses the definite article more often than English does. So los prismáticos is the standard way to say the binoculars.

If you remove the article and say just Prismáticos están aquí, it sounds incomplete or unnatural in standard Spanish.

You could change it if the meaning changes, for example:

• Unos prismáticos están aquí = some binoculars are here
• Mis prismáticos están aquí = my binoculars are here
• Estos prismáticos están aquí = these binoculars are here

Yes, but usually not with the meaning of binoculars. In everyday Spanish, when referring to the object you use to look far away, the normal form is the plural prismáticos.

The singular prismático can exist in other contexts, for example as an adjective or in technical meanings, but for the everyday item, learners should think of it as normally plural:

• los prismáticos

It is neutral and works in both formal and informal situations. There is nothing especially casual or especially formal about it.

You could use it:

• in conversation
• in a shop
• while travelling
• when helping someone find something

It is a simple, standard sentence.