Teman saya bekerja di bidang keuangan.

Yes. In Indonesian, possession is usually noun + possessor, the reverse of English.

• teman = friend
• saya = I / me
• teman saya = my friend (literally: friend I)

Some more examples:

• rumah saya = my house
• mobil dia = his / her car
• guru kami = our teacher

So you don’t say saya teman for my friend; it must be teman saya.

By itself, teman saya is number-neutral. It can mean:

• my friend (singular), or
• my friends (plural), depending on context.

If you really want to make it clearly plural, you can say:

• teman-teman saya = my friends (definitely plural)
• para teman saya (more formal) = my friends

For clearly singular, people often rely on context or add something like:

• satu teman saya = one friend of mine
• seorang teman saya = a (certain) friend of mine

Indonesian normally does not use a verb like to be before verbs or adjectives in the present tense.

So:

• Teman saya bekerja. = My friend works / is working.
  (literally: My friend work)
• Teman saya pintar. = My friend is smart.
  (literally: My friend smart)

You don’t say: Teman saya adalah bekerja or Teman saya adalah pintar.
Adalah is used in more limited, often formal situations, mainly for linking two nouns:

• Dia adalah dokter. = He / she is a doctor.

Indonesian verbs don’t change form for tense. Bekerja is a neutral verb form:

• Teman saya bekerja di bidang keuangan.
  can mean:
  • My friend works in finance. (present, habitual)
  • My friend is working in finance. (present, ongoing)
  • My friend worked in finance. (past, if context is past)

To show time more clearly, Indonesian uses time words:

• kemarin teman saya bekerja di bidang keuangan
  = yesterday my friend worked in finance
• sekarang teman saya bekerja di bidang keuangan
  = now my friend works / is working in finance
• besok teman saya akan bekerja di bidang keuangan
  = tomorrow my friend will work in finance

Both relate to work, but:

• bekerja = to work (full verb, more standard)
• kerja = work; also used as a verb in casual speech

Examples:

• Standard / neutral:

  • Saya bekerja di Jakarta. = I work in Jakarta.

• Informal speech (especially spoken):

  • Saya kerja di Jakarta. = I work in Jakarta.

As a noun, kerja appears in compounds:

• pekerjaan = job, occupation, work (as a noun)
• hari kerja = workday

In your sentence, bekerja is the more neutral, correct verb form.

Breakdown:

• di = in / at / on (location or domain)
• bidang = field / area / sector
• keuangan = finance / finances

So di bidang keuangan literally means in the field of finance.

Saying di keuangan is unusual and sounds incomplete or unnatural. Keuangan is an abstract noun; bidang (field/area) helps show we’re talking about a professional field or sector.

Similar patterns:

• di bidang pendidikan = in the field of education
• di bidang teknologi = in the field of technology

Yes, there are natural alternatives, with slightly different nuance:

• Teman saya bekerja di sektor keuangan.
  = My friend works in the financial sector.
• Teman saya bekerja di dunia keuangan.
  = My friend works in the world of finance. (more figurative)

Bidang is very common and neutral in everyday language, especially when talking about someone’s area of work or expertise. Sektor sounds a bit more technical or economic.

Keuangan comes from:

• uang = money
• ke- … -an = a common noun-forming pattern

So keuangan literally relates to money matters, and usually means:

• finance,
• financial affairs, or in some contexts finances.

Examples:

• laporan keuangan = financial report
• manajer keuangan = financial manager
• departemen keuangan = finance department

Di usually marks location or domain:

• di rumah = at home
• di kantor = at the office
• di bidang keuangan = in the field of finance (seen as a “place” or domain)

Dalam focuses more on inside / within something, often more abstract or formal:

• dalam kotak = inside the box
• dalam konteks ini = in this context
• dalam bidang keuangan can also occur, but sounds more formal or written; di bidang keuangan is more natural in everyday speech.

For talking about which field/sector you work in, di bidang … is the default pattern.

Indonesian has no articles like a, an, or the.

• teman saya can mean my friend or the friend of mine, depending on context.
• bidang keuangan can be understood as a finance field, the finance field, or just finance as a field, again from context.

You don’t add a special word for the. Context, or extra description, does the job if needed.

The sentence uses a very regular Subject–Verb–(Place/Field) order:

• Teman saya (subject)
• bekerja (verb)
• di bidang keuangan (prepositional phrase)

This mirrors English: My friend works in finance.

You generally cannot rearrange it arbitrarily. For example:

• Bekerja teman saya di bidang keuangan
  → wrong / very unnatural in standard Indonesian.

You can sometimes front the place/field for emphasis or style:

• Di bidang keuangan, teman saya bekerja.
  = In the field of finance, my friend works. (unusual in everyday speech, more like written/contrastive emphasis)

Yes, but the pronoun changes the level of formality and region/style:

• saya = formal / polite, neutral; used in most formal situations and with people you don’t know well.
• aku = informal but still polite; used with friends, family, or peers; common in many regions.
• gue / gua = very informal, Jakarta slang; used among close friends, casual situations.

So:

• Teman saya bekerja di bidang keuangan.
  = polite/neutral, good for almost all contexts.
• Teman aku kerja di bidang keuangan.
  = casual, friendly; also note the more casual verb kerja.
• Temen gue kerja di bidang keuangan.
  = very casual Jakarta-style speech; temen is an informal variant of teman.

For learners, saya is the safest default in most situations.