puro ni kiita.

プロ is a shortened katakana word meaning professional.

In Japanese, プロ is commonly used on its own to mean:

• a professional
• an expert
• someone highly skilled in a field

It is borrowed from the English word pro, but in Japanese it behaves like a normal noun.

Depending on context, it could mean:

• a professional person in general
• a specialist
• someone working professionally in a certain area

Katakana is often used for:

• loanwords
• foreign-derived words
• emphasis
• modern or technical vocabulary

プロ is a borrowed word, so katakana is the normal spelling.

Even though it comes from English, it is now a very common Japanese word.

Here, に marks the person that the action is directed toward.

With 聞く, に often marks:

• the person you ask
• the source you hear from

So in this sentence, プロに means something like:

• to a professional
• from a professional

Which one feels more natural depends on the intended meaning of 聞いた in context.

Because プロ is not the direct object here. It is the target/source person of the verb 聞く.

A very common pattern is:

• 人に聞く = to ask a person / hear from a person
• 物を聞く = to hear something

For example:

• 先生に聞く = ask the teacher
• 音楽を聞く = listen to music

So in プロに聞いた, the professional is the person being asked or the person information came from, not the thing being heard.

聞いた is the past plain form of 聞く.

Breakdown:

• dictionary form: 聞く
• past plain form: 聞いた

This is an -u verb transformation:

• 聞く → 聞いた

It usually means:

• asked
• heard
• listened

The exact English translation depends on context.

It can mean both, which is why context matters.

Common meanings of 聞く include:

• to ask
• to hear
• to listen

In a pattern like 人に聞く, it often means:

• to ask someone or
• to hear from someone

So プロに聞いた could be understood as:

• I asked a professional or
• I heard it from a professional

Usually the surrounding context tells you which meaning is intended.

The subject is omitted, which is extremely common in Japanese.

Japanese often leaves out information that is understood from context, such as:

• the subject
• the object
• the topic

So プロに聞いた。 could mean:

• I asked a professional.
• We asked a professional.
• He asked a professional.

In most learning materials, it is often translated as I asked a professional, but grammatically the sentence itself does not explicitly say I.

Yes, the content of what was asked or heard is omitted.

Japanese often leaves out the object when it is already known from context.

For example, a longer sentence could be:

• その問題についてプロに聞いた。 = I asked a professional about that problem.
• 答えをプロに聞いた。 = I asked a professional for the answer.

But if everyone already knows what the topic is, Japanese can simply say:

• プロに聞いた。

So the sentence is complete, even though English often prefers to state more explicitly what was asked.

Yes, プロ is common and natural, but it is somewhat casual and conversational.

More formal or specific alternatives might include:

• 専門家 = specialist, expert
• 専門の人 = a person with expertise
• a job title such as 医者, 弁護士, 整備士, etc.

For example:

• 専門家に聞いた。 = I asked an expert.
• 医者に聞いた。 = I asked a doctor.

So プロに聞いた sounds natural, but it is not the most formal way to say it.

Because Japanese does not need to state the topic or subject in every sentence.

A full sentence could be something like:

• 私はプロに聞いた。
• 彼がプロに聞いた。

But in normal conversation, if the topic is already clear, those parts are often dropped.

That makes プロに聞いた。 perfectly natural as a short, context-dependent sentence.

プロを聞いた would not normally make sense if プロ means professional person.

That is because:

• に marks the person you ask or hear from
• を usually marks the thing you hear/listen to

So:

• プロに聞いた = asked a professional / heard from a professional
• 音を聞いた = heard a sound
• 話を聞いた = listened to a story / heard what someone said

A person with 聞く is usually marked by に, not を, when that person is the source or target of the asking/hearing.

Both can be possible, but the nuance is a little different.

• プロに聞いた often focuses on asking the professional
• プロから聞いた often focuses on hearing it from the professional

So:

• プロに聞いた = I asked a professional / I consulted a professional
• プロから聞いた = I heard it from a professional

In real usage, the difference can be small, but に tends to feel more natural when the idea is asking someone, while から more clearly emphasizes the source of information.