A operação militar começou.

Because a is the feminine singular definite article, meaning the.

In A operação militar começou, the noun operação is feminine singular, so it takes a:

• a operação = the operation

Portuguese articles have to agree with the noun’s gender and number:

• o = masculine singular
• a = feminine singular
• os = masculine plural
• as = feminine plural

So you say a operação, not o operação.

Grammatical gender in Portuguese does not always match the ending in a predictable way.

Although many feminine nouns end in -a, some feminine nouns have other endings, including -ão. Operação is one of them, and you simply learn it as a feminine noun:

• a operação
• esta operação
• uma operação militar

A useful clue is that many nouns ending in -ção are feminine:

• a nação
• a informação
• a decisão
• a operação

So if you see -ção, it is very often feminine.

Here, militar is an adjective.

It describes the noun operação, so operação militar means military operation.

In Portuguese, militar can also be a noun in other contexts:

• um militar = a member of the military, a serviceman / military person

But in this sentence, it is clearly adjectival because it modifies operação.

Because in Portuguese, adjectives often come after the noun.

So:

• operação militar
• decisão importante
• zona central

This is one of the most common word orders in Portuguese. Some adjectives can come before the noun, but many are normally placed after it, especially more factual or classifying adjectives like militar.

So operação militar sounds natural, while putting militar before the noun would sound unusual in ordinary usage.

Começou is the pretérito perfeito do indicativo in Portuguese, usually translated as the simple past in English.

It is the he/she/it form of the verb começar:

• eu comecei
• tu começaste
• ele/ela/você começou
• nós começámos (European Portuguese spelling and pronunciation)
• vocês/eles/elas começaram

In this sentence, a operação militar is singular, so the verb is in the singular third person:

• A operação militar começou.

Because the subject is singular.

The subject is A operação militar, which is one operation, not several. So the verb must also be singular:

• A operação militar começou. = singular
• As operações militares começaram. = plural

Portuguese verbs must agree with the subject in number and person.

The ç is called cê-cedilha.

It shows that the c is pronounced like s, not like k.

So:

• começou sounds with an s sound in that part
• without the cedilla, c before a, o, or u would normally sound like k

This is why Portuguese writes:

• começar
• começou

But not comecar or comecou.

The accent in começou shows the stress pattern.

The stressed syllable is the last one:

• co-me-ÇOU

In Portuguese spelling, accents often help show which syllable is stressed and sometimes also the vowel quality.

So the accent is not optional here; it is part of the correct spelling.

The main thing to remember is that the stress falls on the last syllable:

• o-pe-ra-ÇÃO

A few pronunciation points for European Portuguese:

• ção has a nasal sound
• the final -ão is nasal, not a plain ow
• unstressed vowels are often reduced in European Portuguese

A rough learner-friendly guide is:

• operação ≈ oh-peh-ruh-SOUNG / oh-peh-ruh-SOW̃

Not exact English, but enough to point you in the right direction. The most important thing is the final stressed nasal -ção.

The stress is on the last syllable:

• co-me-ÇOU

A rough guide is:

• começou ≈ kuh-meh-SOH

In European Portuguese, unstressed vowels are often weaker than in English, so the first syllables may sound less clear than you expect. The final ou is usually pronounced like a closed o sound in standard European Portuguese, not a strong English ow.

So focus on:

• soft first syllables
• stress on çou
• ç pronounced like s

In this sentence, começou is the normal Portuguese way to express a completed past event, and in many contexts it corresponds to English began.

So:

• A operação militar começou. = The military operation began.

Portuguese often uses the simple past (pretérito perfeito) where English also uses the simple past. It does not work exactly like the English present perfect has begun.

If you are translating into natural English, began is usually the best match here.

Yes. That is perfectly correct if the context already makes it clear what kind of operation you mean.

• A operação começou. = The operation began.
• A operação militar começou. = The military operation began.

Adding militar makes the noun phrase more specific.

The plural would be:

• As operações militares começaram.

Here is what changes:

• a → as
• operação → operações
• militar → militares
• começou → começaram

Everything has to agree properly in number.

Normally, no—not in standard neutral Portuguese.

Portuguese uses definite articles more often than English does, and with a subject like this, a is expected:

• A operação militar começou.

Without the article, it sounds incomplete or unnatural in ordinary standard usage, unless it is a headline, note, or some very compressed style.

So for normal sentences, keep the article.