Я был в этом музее только один раз.

Был is the past tense of быть (to be). In this sentence, it means was.

So Я был в этом музее literally means I was in this museum, which is how Russian commonly expresses I have been to this museum in English.

A useful thing to remember: Russian often uses the simple past where English might use the present perfect.

In the past tense, Russian verbs agree with the gender of the subject in the singular.

• Я был = said by a man
• Я была = said by a woman

So:

• Я был в этом музее только один раз. = a male speaker
• Я была в этом музее только один раз. = a female speaker

Because this sentence describes location, not movement.

• в этом музее = in this museum / at this museum → location
• в этот музей = into this museum / to this museum → direction or motion

Compare:

• Я был в этом музее. = I was in this museum.
• Я пошёл в этот музей. = I went to this museum.

So after в, Russian uses different cases depending on meaning:

• location → Prepositional case
• movement toward → Accusative case

Because after в meaning in (location), Russian uses the Prepositional case.

That changes the forms:

• этот → этом
• музей → музее

So:

• dictionary form: этот музей = this museum
• after в for location: в этом музее = in this museum

This is a very common pattern:

• в этом городе = in this city
• в этом доме = in this house
• в этом музее = in this museum

It means only once or more literally only one time.

Breakdown:

• только = only
• один = one
• раз = time / occasion

So один раз is a standard way to say once in Russian.

Examples:

• Я видел его один раз. = I saw him once.
• Я был там только один раз. = I’ve only been there once.

Раз is commonly used when counting how many times something happened.

Examples:

• один раз = once
• два раза = twice / two times
• три раза = three times

So in this sentence, раз means occasion or time in the sense of number of visits.

It does not mean time as in clock time. For that, Russian would use время.

Yes. Russian word order is fairly flexible, and changing it often changes the emphasis, not the basic meaning.

The original sentence:

• Я был в этом музее только один раз.

Other possible versions:

• Я только один раз был в этом музее.
• В этом музее я был только один раз.
• Только один раз я был в этом музее.

All of these can mean roughly the same thing, but the focus shifts:

• В этом музее first = emphasis on this museum
• Только один раз first = emphasis on only once

The original version is very natural and neutral.

Usually yes, or at least it is very helpful.

In the Russian past tense, был by itself does not show person clearly:

• был could mean I was or he was, depending on context
• была could mean I was or she was

So Я makes the subject explicit.

In conversation, Russian can omit pronouns when the meaning is obvious from context, but in a standalone sentence, Я is normally included.

Russian usually does not make the same present perfect vs. simple past distinction that English does.

English often says:

• I have been to this museum only once.

Russian normally says:

• Я был в этом музее только один раз.

So even though the English translation may use have been, Russian uses the ordinary past tense form был.

This is completely normal and very common.

Yes. Только can move, and where you place it affects what sounds most emphasized.

Compare:

• Я был в этом музее только один раз.
  Neutral: I was in this museum only once.

• Я только один раз был в этом музее.
  Stronger emphasis on only once.

• Я был только в этом музее один раз.
  This could sound like emphasis on only in this museum, so it may suggest something slightly different.

So while Russian word order is flexible, только should be placed carefully because it tends to attach itself to whatever follows or whatever is being focused on.

If the subject were we, for example, the past tense form would change:

• Мы были в этом музее только один раз. = We were / have been to this museum only once.

Past tense agreement works like this:

• был = masculine singular
• была = feminine singular
• было = neuter singular
• были = plural

So the verb changes depending on who the subject is.