El tema está bien explicado.

Because estar is used here to describe the state or condition of the topic: it is well explained.

• El tema está bien explicado = the topic is in a well-explained state
• ser + past participle is more typical for a true passive construction, especially when you mention the agent:
  • El tema fue explicado por la profesora = the topic was explained by the teacher

In everyday Spanish, estar + past participle is very common when the focus is on the result, not on who did the action.

Explicado is the past participle of explicar (to explain).

Here it works like an adjective, describing el tema:

• explicar → infinitive
• explicado → past participle
• bien explicado → well explained

So in this sentence, explicado is not acting like a full verb by itself. It is part of the structure estar + past participle.

It is close to a passive idea, but learners are usually helped most by thinking of it as a resulting-state construction.

• El tema está bien explicado focuses on the current state of the topic.
• A more clearly passive sentence would be:
  • El tema fue bien explicado
  • El tema es bien explicado is grammatically possible in theory, but it sounds unnatural in most everyday contexts.

So the sentence is best understood as: the topic is well explained, with attention on the result.

Because bien is an adverb, and it modifies explicado.

• bien = well
• bueno = good

In English, we also say well explained, not good explained.

Compare:

• Está bien explicado = it is well explained
• Es bueno = it is good

So bien is the correct choice because it tells us how it is explained.

Tema often means topic, subject, or theme, depending on context.

In this sentence, the most natural meaning is usually:

• topic
• sometimes subject matter

A native English speaker might first think of theme, because tema looks similar, but in many everyday learning contexts topic is the best translation.

Spanish often uses the definite article el / la / los / las in places where English might or might not use the.

Here, el tema means the topic. It refers to a specific topic already understood from context.

If you just said Tema está bien explicado, that would be ungrammatical. Spanish normally needs an article or another determiner here:

• El tema está bien explicado
• Este tema está bien explicado
• Ese tema está bien explicado

Yes. You can add the agent, although this sentence usually leaves it out because the result matters more than the person.

For example:

• El tema está bien explicado por la profesora.

That said, Spanish often prefers other ways of expressing this if you want to emphasize the person:

• La profesora explica muy bien el tema.
• La profesora ha explicado bien el tema.

So adding por... is possible, but not always the most natural choice.

Yes. Past participles used adjectivally agree in gender and number with the noun they describe.

Here:

• tema is masculine singular
• so we use explicado

Compare:

• El tema está bien explicado.
• La lección está bien explicada.
• Los temas están bien explicados.
• Las lecciones están bien explicadas.

This agreement is very important.

The accent mark in está shows both stress and distinguishes it from esta.

• está = is (from estar)
• esta = this (feminine), as in esta casa

So the accent is not optional. It changes the word.

You may hear adverbs placed after participles sometimes, but bien explicado is the most natural and standard order here.

• El tema está bien explicado = the usual, natural phrasing

Putting bien before explicado makes the phrase feel like a single descriptive unit: well explained.

So for a learner, the safest and best version is:

• El tema está bien explicado.

A simple pronunciation guide is:

el TE-ma es-TA byen eks-pli-KA-do

A few details:

• tema = TE-ma
• está = stress on the last syllable: es-TA
• bien sounds roughly like byen
• explicado = eks-pli-KA-do

In most of Spain:

• e and o are pure vowels, shorter and cleaner than in English
• b in bien is a soft Spanish b/v sound, not exactly like a strong English b

Yes, it is completely natural.

It sounds like something you might say when talking about:

• a textbook
• a lesson
• a presentation
• an article
• study materials

For example:

• El tema está bien explicado en el libro.
• No te preocupes, el tema está bien explicado.

So this is a very useful and idiomatic sentence pattern to learn.