El ejército cruzó la frontera.

Because ejército is a masculine noun, so it takes el in the singular.

• el ejército = the army
• plural: los ejércitos = the armies

Its gender is something you simply learn with the noun. In this case, it helps that the word ends in -o, which is often masculine, though that is not a perfect rule in Spanish.

Cruzó is the preterite form of cruzar for él/ella/usted.

So:

• cruzar = to cross
• cruzó = he crossed / she crossed / it crossed / you crossed (formal)

In this sentence, el ejército is singular, so the verb is in the third person singular: cruzó.

The preterite is used here because it describes a completed action in the past: the army crossed the border.

The accent mark shows both stress and meaning.

• cruzo = I cross
• cruzó = he/she/it crossed

So the accent is very important. Without it, the form would be different.

It also tells you the stress falls on the final syllable: cru-ZÓ.

Because Spanish spelling rules require it here to show the correct stress.

Ejército is stressed on the first syllable: E-jér-ci-to.

Without the accent, Spanish readers would expect a different stress pattern. The written accent tells you exactly where the stress goes.

Because Spanish often omits subject pronouns when the subject is already clear.

Here, the subject is explicitly stated as el ejército, so there is no need to add a pronoun.

Spanish prefers:

• El ejército cruzó la frontera.

not:

• El ejército él cruzó la frontera. ❌

Also, even if the noun were not present, Spanish often leaves out the pronoun because the verb form already gives information about the subject.

Spanish uses the definite article more often than English.

So where English might say crossed the border, Spanish naturally says cruzó la frontera.

Using la makes it sound complete and natural in Spanish. Without the article, cruzó frontera, it would sound wrong in standard Spanish.

Also, frontera is a feminine singular noun, so it takes la.

In standard Peninsular Spanish, a rough pronunciation would be:

el eh-HER-see-toh kroo-THOH lah fron-TEH-rah

A few useful points:

• ejército: the j is a strong throaty sound, not like English j
• cruzó: in much of Spain, z is pronounced like English th in think
• frontera: the r here is a light tap, not a strong English r

A more phonetic version for Spain would be approximately:

[el eˈxerθito kruˈθo la fɾonˈteɾa]

Because cruzar usually takes a direct object in Spanish.

So you say:

• cruzar la frontera = to cross the border
• cruzar la calle = to cross the street
• cruzar el río = to cross the river

This is similar to English, where cross also takes a direct object.

So El ejército cruzó la frontera is structured very naturally:

• subject: El ejército
• verb: cruzó
• direct object: la frontera

Yes, you could. Atravesó from atravesar can also mean crossed.

So both are possible:

• El ejército cruzó la frontera.
• El ejército atravesó la frontera.

But cruzar is the most straightforward and common choice for crossing a border, street, river, and so on.

Atravesar can sometimes feel slightly more like go across or pass through, depending on context, but in many cases the difference is small.

Yes, Spanish word order is somewhat flexible, though the neutral order here is:

• El ejército cruzó la frontera.

You might also see:

• Cruzó la frontera el ejército.

But that sounds more marked or literary, and it may put extra focus on el ejército.

For a learner, the standard order subject + verb + object is the safest and most natural choice.

Usually ejército means army. In many contexts, it refers specifically to the land army.

If you want to say the armed forces or the military more generally, Spanish often uses:

• las fuerzas armadas = the armed forces
• los militares = the military / military personnel

So in this sentence, el ejército is best understood as the army.

Yes. Grammatically, ejército is a singular collective noun.

So even though it refers to many soldiers, Spanish treats it as a singular subject:

• El ejército cruzó la frontera. ✅

Not:

• El ejército cruzaron la frontera. ❌

This is similar to English using the army crossed the border. The group contains many people, but the noun itself is singular.