Мы обратились в бюро за помощью.

Обратились is the perfective past form, so it presents the action as a completed event: we turned/applied/reached out to the bureau.

By contrast, обращались is imperfective. It would usually suggest repeated action, process, or general experience, for example:

• Мы часто обращались в бюро за помощью. = We often went to the bureau for help.
• Мы уже обращались в бюро. = We have already contacted the bureau before.

So in your sentence, обратились fits a single completed action.

The -сь is the reflexive particle (a variant of -ся). In this verb, it is simply part of the dictionary form обратиться.

For English speakers, it is best not to think of it as always meaning a literal reflexive like wash oneself. In Russian, many verbs just normally exist with -ся / -сь, and the meaning can be idiomatic.

Here, обратиться means to turn to someone/something, to apply, or to seek help/advice from.

So:

• обратить = to turn/direct something
• обратиться = to turn to, to appeal to, to apply to

Because the subject is мы = we.

In the past tense, Russian verbs agree with the subject in number and, in the singular, also in gender:

• я обратился / обратилась = I turned/applied
• он обратился = he turned/applied
• она обратилась = she turned/applied
• оно обратилось = it turned/applied
• мы обратились = we turned/applied
• они обратились = they turned/applied

So -ли marks the plural past tense.

Here в means to / into in the sense of direction toward a place or institution, so Russian uses the accusative case.

With many nouns, you would see a visible accusative ending after в when there is movement:

• в школу = to school
• в банк = to the bank
• в офис = to the office

In в бюро, the noun бюро is an indeclinable noun, so its form does not change. Even though the case is accusative here, it looks exactly the same.

Because бюро is an indeclinable noun. Many borrowed words in Russian do not change form across cases.

So you may see:

• в бюро
• из бюро
• о бюро

The case is understood from the preposition and the role in the sentence, not from a changed ending.

This is very common with certain loanwords such as:

• метро
• пальто
• кафе
• кино

Помощью is the instrumental case of помощь.

The expression обратиться за чем-либо means to turn/apply for something. After за in this meaning, Russian uses the instrumental:

• за помощью = for help
• за советом = for advice
• за информацией = for information
• за поддержкой = for support

So this is a fixed and very common pattern:

• обратиться за помощью = to seek help
• обратиться за советом = to ask for advice

Because after за in the sense of for / in search of / to obtain, Russian uses the instrumental case, not the genitive.

The noun помощь changes like this:

• nominative: помощь
• genitive: помощи
• dative: помощи
• accusative: помощь
• instrumental: помощью
• prepositional: помощи

So in за помощью, the ending -ью shows the instrumental singular.

It can mean either, depending on context.

In Russian, обратиться в + institution/organization often means to contact, to apply to, or to turn to that institution for assistance, not necessarily to walk into the building physically.

So Мы обратились в бюро за помощью can mean:

• we went to the bureau for help,
• we applied to the bureau for help,
• we contacted the bureau for help.

The exact nuance depends on context.

Yes, Russian word order is fairly flexible.

This sentence is neutral and natural:

• Мы обратились в бюро за помощью.

But you could also say:

• За помощью мы обратились в бюро.
• В бюро мы обратились за помощью.

These versions shift emphasis:

• За помощью first emphasizes what was sought.
• В бюро first emphasizes where they turned.

English relies more on word order for grammar, but Russian often uses endings and prepositions for grammar, so word order is more about focus and style.

The stress is:

• мы обратИлись в бюрО за пОмощью

More specifically:

• обратИлись
• бюрО
• пОмощью

Stress is important in Russian because it is not always predictable and can affect pronunciation a lot.

Usually, no—not in this sentence.

With institutions, Russian commonly says обратиться в when meaning to apply to / contact an organization:

• обратиться в суд = to apply to the court
• обратиться в полицию = to go to/contact the police
• обратиться в банк = to apply to the bank

К is more often used with people or sometimes in other contexts:

• обратиться к врачу = to turn to a doctor
• обратиться к другу = to turn to a friend
• обратиться к специалисту = to turn to a specialist

So:

• в бюро = to the bureau/agency
• к сотруднику бюро = to an employee of the bureau

Yes, бюро is normally treated as neuter singular.

Even though it does not decline, it still has grammatical gender, and that can matter in agreement, for example:

• Это бюро открыто. = This bureau is open.
• Новое бюро находится рядом. = The new bureau is nearby.

So it is indeclinable, but not genderless.

Yes. За помощью is a very common phrase and can be used with other verbs of motion or seeking:

• идти за помощью = to go for help
• прийти за помощью = to come for help
• звонить за помощью is less natural, but звонить, чтобы попросить помощи would work depending on context
• обратиться за помощью = to seek / ask for help

Among these, обратиться за помощью is one of the most standard and useful expressions to remember.