Hacer pleno significa acertar todas las preguntas.

Because this sentence is giving a definition.

In Spanish, an infinitive can work like a noun:

• Hacer pleno = getting a full score / getting them all right
• Acertar todas las preguntas = getting all the questions right

So the structure is:

[infinitive phrase] + significa + [infinitive phrase]

This is very common in definitions, for example:

• Estudiar significa aprender.
• Mentir significa decir algo falso.

The whole phrase hacer pleno is treated as one singular idea, which is why the verb is significa and not plural.

It is mostly an idiomatic expression. Literally, hacer means to do/make, and pleno means something like full or complete, but the phrase hacer pleno does not translate word for word naturally into English.

In context, it means:

• to get everything right
• to score full marks
• to make a clean sweep

It is often used in quizzes, tests, games, betting, or any situation where someone gets all possible points or answers correct.

So it is better to learn hacer pleno as a set expression, not by translating each word separately.

In this expression, it is best to think of pleno as part of a fixed phrase: hacer pleno.

On its own, pleno can be:

• an adjective meaning full
• a noun in some contexts, such as in politics or institutions: el pleno del ayuntamiento

But in hacer pleno, what matters most is the idiomatic combination. For a learner, the safest approach is:

• learn hacer pleno as one chunk
• understand it as to get everything right / achieve the maximum

Because hacer pleno is commonly used as a set expression without an article.

Spanish often has fixed combinations like this, where English learners may expect an article but Spanish does not use one. In many contexts, hacer pleno sounds more natural than hacer un pleno.

That said, in some situations native speakers may also say hacer un pleno, depending on region, tone, or context, but hacer pleno is a very established expression, especially in Spain.

So for this sentence, hacer pleno without an article is normal and idiomatic.

Because the subject of the sentence is the whole infinitive phrase hacer pleno, and that whole phrase is treated as singular.

Compare:

• Hacer pleno significa...
• Estudiar mucho ayuda...
• Fumar es malo...

Even though the phrase may contain several words, it still counts grammatically as one thing.

Here, acertar means to get right or to answer correctly.

Examples:

• Acerté la respuesta. = I got the answer right.
• Acertó todas las preguntas. = He/She got all the questions right.

This verb is especially common for:

• quiz answers
• exam questions
• guesses
• predictions
• choices among several options

So in this sentence, acertar todas las preguntas means to answer all the questions correctly.

This is a very common question.

• acertar = to get right
• adivinar = to guess

So:

• If you choose the correct answer, you aciertas.
• If you do not know and you just guess, you adivinas.

Sometimes a guess can also be correct, so both ideas can overlap in real life. But the focus is different:

• acertar focuses on the correct result
• adivinar focuses on the fact that it was a guess

Example:

• Adiviné la respuesta y la acerté.
  • I guessed the answer and got it right.

Because acertar takes a direct object.

So you say:

• acertar todas las preguntas
• acertar la respuesta
• acertar el resultado

There is no personal a here, because preguntas are things, not people.

This is different from verbs like responder or contestar, which often use a:

• responder a todas las preguntas
• contestar a todas las preguntas

So:

• acertar todas las preguntas = to get all the questions right
• responder a todas las preguntas = to answer all the questions

Those are not exactly the same meaning.

Because in Spanish, when you mean all the ..., you normally use:

todo/toda/todos/todas + definite article + noun

So:

• todas las preguntas = all the questions
• todos los días = every day / all the days
• todas las respuestas = all the answers

The words must agree:

• todas because preguntas is feminine plural
• las because preguntas is feminine plural

So the full pattern is important:

• todas las preguntas not
• todas preguntas in normal standard usage here

Yes, and it is a very natural alternative.

Compare:

• acertar todas las preguntas = to get all the questions right
• responder bien a todas las preguntas = to answer all the questions correctly

They are very similar in meaning here.

However, acertar is a bit more compact and directly emphasizes being correct.
Responder bien a todas las preguntas is slightly more explicit.

Another good alternative would be:

• contestar bien a todas las preguntas

Yes, hacer pleno is particularly common in Spain.

A learner of Spanish from Spain should definitely recognize it. In other Spanish-speaking regions, people may understand it, but they might prefer other expressions depending on context, such as:

• sacar la puntuación máxima
• obtener todos los puntos
• responder todo correctamente

So if your focus is Spanish from Spain, hacer pleno is a very useful expression to know.

Yes, but the original version is the most neutral for a definition:

• Hacer pleno significa acertar todas las preguntas.

You could also say:

• Acertar todas las preguntas significa hacer pleno.

That is grammatically correct, but it changes the focus slightly. It sounds like you are defining acertar todas las preguntas instead.

For dictionary-style explanations or textbook-style definitions, the original order is the most natural:

• term being defined
  • significa
    • definition

Yes, but it sounds a little like an explanation or definition, not casual conversation.

A native speaker might say it when:

• explaining a phrase
• talking about a game or quiz
• clarifying what counts as a perfect result

In more everyday speech, you might hear:

• Ha hecho pleno.
• Hiciste pleno.
• Si aciertas todas las preguntas, haces pleno.

So the sentence you have is perfectly natural, but it has a slightly teaching/explaining tone.