El uniforme militar está limpio.

Because uniforme is a masculine noun in Spanish: el uniforme.

A common mistake is to assume noun gender from the ending, but nouns ending in -e can be either masculine or feminine, so you usually have to learn the article with the noun.

Examples:

• el uniforme
• el coche
• la clase
• la noche

So in this sentence, el is simply the correct masculine article for uniforme.

Here, militar is an adjective meaning military.

So:

• uniforme = uniform
• uniforme militar = military uniform

In Spanish, many adjectives come after the noun, so uniforme militar is the normal order.

Also, militar does not change for masculine vs feminine:

• el uniforme militar
• la chaqueta militar

But it does change for plural:

• los uniformes militares
• las chaquetas militares

Spanish uses both ser and estar for to be, but they are used differently.

Here, está limpio means the uniform is clean right now / in its current state. Cleanliness is seen as a condition, so estar is the normal choice.

• está limpio = it is clean
• está sucio = it is dirty

If you said es limpio, it would usually suggest a more general characteristic, such as it is a clean thing by nature or it is tidy/hygienic in general, which is less natural in this context.

So for temporary or observable states like clean/dirty, estar is usually the right verb.

Because the él/ella/usted form of estar in the present tense is spelled está.

The accent shows the stress falls on the final syllable:

• es-TÁ

It also helps distinguish it from esta, which can be a demonstrative word meaning this:

• esta chaqueta = this jacket

So:

• está = is
• esta = this

Because limpio agrees with uniforme, and uniforme is masculine singular.

Spanish adjectives usually agree in gender and number with the noun they describe.

Here:

• uniforme = masculine singular
• so the adjective is limpio = masculine singular

Compare:

• El uniforme está limpio.
• La camisa está limpia.
• Los uniformes están limpios.
• Las camisas están limpias.

Because in Spanish, descriptive adjectives often come after the noun.

So:

• uniforme militar
• coche rojo
• casa grande

That is the normal pattern in Spanish, even though English usually puts adjectives before the noun:

• military uniform
• red car
• big house

Some adjectives can go before the noun in Spanish, but that often changes the tone or meaning. With militar, the normal order here is definitely uniforme militar.

Not in normal Spanish. El militar uniforme sounds unnatural and incorrect in this context.

The natural order is:

• El uniforme militar está limpio.

This is because militar is classifying the type of uniform, and those kinds of adjectives normally go after the noun.

Here, limpio is an adjective meaning clean.

That matters because limpio can also be the yo form of the verb limpiar:

• Yo limpio el uniforme. = I clean the uniform.

But in your sentence:

• El uniforme militar está limpio.

it comes after está, so it is clearly an adjective describing the uniform.

Yes, in a normal sentence like this, el is needed.

Spanish usually requires an article with a singular countable noun when you are talking about a specific item:

• El uniforme militar está limpio.

Without the article:

• Uniforme militar está limpio

that would sound incomplete or unnatural in ordinary speech.

A simple pronunciation guide is:

el oo-nee-FOR-meh mee-lee-TAR es-TA LEEM-pyoh

Stress:

• uniFORme
• miliTAR
• esTÁ
• LIMpio

A few useful notes:

• u sounds like oo
• r in militar is a light Spanish r
• ll is not present here, so everything is fairly straightforward for an English speaker

In the plural, both the noun phrase and the adjective have to agree:

• Los uniformes militares están limpios.

Changes:

• el → los
• uniforme → uniformes
• militar → militares
• está → están
• limpio → limpios

This is a good example of how Spanish agreement works across the whole sentence.