A teoria explica o problema.

Portuguese uses definite articles very often, more often than English does.

• a = the for a feminine singular noun
• o = the for a masculine singular noun

So:

• a teoria = the theory
• o problema = the problem

In this sentence, the articles make the nouns sound specific. Leaving them out here would sound unnatural in normal Portuguese.

Because teoria is a feminine noun in Portuguese, so it takes the feminine article a.

A lot of nouns ending in -a are feminine, and teoria follows that pattern:

• a teoria
• a história
• a ideia

The gender is grammatical, so adjectives and articles must agree with it.

This is a very common learner question. Not every noun ending in -a is feminine.

Problema is a masculine noun, so it takes o:

• o problema

Many Portuguese nouns of Greek origin ending in -ma are masculine, for example:

• o problema
• o sistema
• o tema
• o programa

So this is one of those nouns you simply need to learn with its article.

Explica is the 3rd person singular present indicative of explicar.

Infinitive:

• explicar = to explain

Present tense:

• eu explico
• tu explicas
• ele / ela / você explica

Here, explica agrees with a teoria, which is singular:

• A teoria explica...

In Portuguese, the simple present can often cover both ideas, depending on context.

So explica can mean:

• explains
• sometimes is explaining

In a sentence like this, without extra context, English usually translates it as explains.

If Portuguese wants to be very explicit about an action happening right now, it often uses:

• estar a + infinitive in European Portuguese

For example:

• A teoria está a explicar o problema
  This would be unusual in meaning here, but grammatically it shows an ongoing action.

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

Here the subject is explicitly stated:

• A teoria

So there is no need to add ela.

You could say:

• A teoria explica o problema
• Ela explica o problema

But if you already say A teoria, adding ela would usually be unnecessary.

Because explicar normally takes a direct object.

So you say:

• explicar algo = to explain something

That is why the sentence is:

• A teoria explica o problema

Not:

• A teoria explica ao problema

However, if you explain something to someone, then you use a preposition:

• explicar algo a alguém
• A professora explica a matéria aos alunos

So:

• o problema = the thing being explained
• a alguém = the person receiving the explanation

Yes, this sentence uses the normal Subject + Verb + Object order:

• A teoria = subject
• explica = verb
• o problema = direct object

This is the most neutral and natural order in Portuguese too.

Portuguese can sometimes change word order for style, emphasis, or literary effect, but this version is the standard one.

Yes. Grammatically, explica can also be used with você:

• Você explica

That means you explain in a formal or neutral register.

But in your sentence, the subject is clearly A teoria, so here explica means explains, not you explain.

In standard Portuguese, you would usually use a direct object clitic pronoun:

• A teoria explica-o.

Here:

• o = him / it as a masculine direct object pronoun
• it refers to o problema

In European Portuguese, these object pronouns are often attached after the verb in affirmative main clauses:

• explica-o

This kind of pronoun placement is very common in Portuguese grammar, even though it may feel unfamiliar to English speakers.

A rough European Portuguese pronunciation is:

• ish-PLI-kɐ

A few useful points:

• the stress is on pli
• the first e is weak
• the x here sounds like sh
• the final a is reduced, not a full strong a like in Spanish

So it does not sound like English ex-פּלიკა. In European Portuguese, it is much more compressed and reduced in unstressed syllables.