El ejército protege la frontera.

Because Spanish articles change for gender and number.

• el = masculine singular
• la = feminine singular

So:

• el ejército because ejército is masculine
• la frontera because frontera is feminine

Yes, ejército is a masculine noun, and nouns ending in -o are often masculine. But that is only a pattern, not a guarantee. The safest habit is to learn nouns together with their article:

• el ejército
• la frontera

Because the stress falls on the third-to-last syllable: e-jér-ci-to.

In Spanish, any word stressed on the third-to-last syllable must have a written accent mark. That is why ejército needs é.

Roughly, in much of Spain, it sounds like eh-HER-thi-toh.

A few key points:

• j is a strong throaty sound, not like English j
• c before i is usually pronounced like th in think in most of Spain
• the stress is on jér

So the stressed part is JÉR.

Roughly fron-TE-ra.

A few details:

• the stress is on te
• the r in frontera is a single r, so it is a light tap, not the strong rolled rr
• the fron- part is similar to English fron, but with Spanish vowels

Protege is the third-person singular present indicative form of proteger.

It matches the subject el ejército, which is singular:

• el ejército protege = the army protects

Because before e and i, Spanish g already gives the right sound for this verb.

The spelling only changes when necessary to keep the pronunciation. For example:

• proteger
• protege
• proteges
• protejo ← here it changes to j before o

If it were protego, the g would sound different, so Spanish writes protejo instead.

Because the personal a is mainly used with people and sometimes animals treated like individuals.

Here, la frontera is a thing, not a person, so there is no personal a:

• El ejército protege la frontera.

But with a person, you would often use a:

• El ejército protege a los civiles.

Sometimes yes, but the meaning becomes less clear.

Spanish often omits the subject when context already makes it obvious. However, with protege la frontera, without context it could mean:

• he protects the border
• she protects the border
• it protects the border
• or even a formal command: Protect the border

So keeping el ejército makes the subject explicit.

Usually it means a specific, understood army, like English the army. But the exact meaning depends on context.

It can refer to:

• a particular country's army
• the army as an institution
• a specific army already mentioned

So the sentence itself is grammatically clear, but context tells you exactly which army is meant.

This is the most neutral and standard word order in Spanish:

• subject + verb + object
• El ejército protege la frontera.

Spanish can change word order for emphasis, but this version is the normal, unmarked one.

Because it follows the normal Spanish stress rules, so it does not need one.

In protege, the stress naturally falls on the second-to-last syllable: pro-te-ge. Words ending in a vowel are normally stressed that way, so no accent mark is required.