Fratele meu este în armată.

Because Romanian often puts the definite article at the end of the noun instead of using a separate word like English the.

• frate = brother
• fratele = the brother

So fratele meu literally means the brother of mine, which is how Romanian normally says my brother.

In Romanian, possessives usually come after the noun when the noun is definite.

So the normal pattern is:

• fratele meu = my brother
• casa mea = my house
• prietenul meu = my friend

This is different from English, where my comes before the noun.

To an English speaker, it can feel redundant, but in Romanian this is the standard structure.

Romanian commonly uses:

• definite noun + possessive

So:

• fratele meu literally = the brother my
• natural English = my brother

You generally do not say meu frate in ordinary neutral speech for this meaning.

Because frate/fratele is a masculine singular noun.

The possessive adjective has to agree with the noun:

• masculine singular: meu
• feminine singular: mea
• masculine plural: mei
• feminine plural: mele

Examples:

• fratele meu = my brother
• sora mea = my sister
• frații mei = my brothers
• surorile mele = my sisters

Este is the 3rd person singular of a fi, meaning to be.

Since the subject is fratele meu = my brother / he, Romanian uses:

• eu sunt = I am
• tu ești = you are
• el/ea este = he/she is

So:

• Fratele meu este în armată. = My brother is in the army.

In everyday speech, este is often shortened to e:

• Fratele meu e în armată.

Both are correct. Este is a bit fuller or slightly more formal.

Normally, no. In standard Romanian, the verb to be is usually expressed in sentences like this.

So you say:

• Fratele meu este în armată.
• Fratele meu e în armată.

But not normally just:

• Fratele meu în armată.

That would sound incomplete in standard Romanian.

Here armată is being used in a general institutional sense, like English in the army.

Romanian often uses the noun without the definite article in this kind of expression:

• în armată = in the army / serving in the military

If you say în armata, that usually needs more information after it, such as:

• în armata română = in the Romanian army
• în armata aceea = in that army

So în armată is the natural form for the general idea.

Yes, you may hear la armată, especially in expressions related to military service or being in the military environment.

But there is a slight nuance:

• în armată = in the army, in the military as an institution
• la armată = at/in the army, often more colloquial or tied to doing military service

For a learner, este în armată is a very safe and standard choice.

After the preposition în, Romanian uses the form that looks like the accusative. In practice, for many nouns, this looks just like the basic dictionary form.

So:

• armată is the singular form of the noun
• after în, it stays armată

You do not need to change it to a special new form here.

A simple learner-friendly pronunciation is:

FRA-te-le meu ES-te în ar-ma-TĂ

A few helpful notes:

• fr in fratele sounds much like English fr in fresh
• eu in meu is pronounced as one syllable, roughly like meh-o said quickly
• în has the Romanian vowel â/î, which does not exist exactly in English
• ă in armată is the short central vowel heard in many unstressed English syllables, somewhat like the a in about

If you want a rougher English-style approximation:

FRA-teh-leh meh-o EH-steh in ar-ma-TUH

The normal, neutral order is:

• Fratele meu este în armată.

Romanian can change word order for emphasis, but not all changes sound equally natural.

For example:

• În armată este fratele meu.
  This is possible, but it sounds marked, as if emphasizing in the army.

However, meu frate is not the usual everyday way to say my brother. The standard form is:

• fratele meu

So as a learner, the safest pattern is:

• noun + definite article + possessive

Yes, in normal Romanian, fratele meu is the standard and most natural expression.

You might also hear:

• un frate de-al meu = a brother of mine

But that means something slightly different. It is indefinite and not the normal direct equivalent of my brother.

So for this sentence, fratele meu is exactly the form you want.