Learn Hardcore Ukrainian: Учора я викликав майстра, бо пральна машина зупинилася. - Yesterday I called a repairman because the washing machine stopped.

If you only mean you dialed them, use зателефонував/подзвонив (майстру/у сервіс) = “phoned (the technician/the service).”
If you mean you booked them to come, викликав майстра is the idiomatic choice.

Why is it майстра, not майстер?

Because it’s a direct object referring to an animate masculine noun. In Ukrainian, the accusative singular of animate masculine nouns looks like the genitive.

Nominative (subject): майстер
Accusative (animate, object): майстра
Compare: бачу брата/вчителя/майстра (“I see the brother/teacher/technician”).

Does викликав mean the speaker is male?

Yes. Past-tense verbs agree with the subject’s gender and number.

Male speaker: я викликав
Female speaker: я викликала
Plural: ми викликали

Why does it say зупинилася and not зупинився?

Because пральна машина is feminine (машина is feminine), so the past-tense verb agrees in gender: зупинилася.

Masculine subject: зупинився
Neuter subject: зупинилося
Plural subject: зупинилися

What does the -ся in зупинилася do?

-ся marks a reflexive/intransitive form. Зупинитися = “to stop (come to a stop).”
Contrast with the transitive зупинити = “to stop (something/someone).”

Машина зупинилася. = “The machine stopped.”
Я зупинив машину. = “I stopped the machine.”
Note: You may also see the short colloquial form зупинилась (with -сь).

Can I use бо, тому що, or оскільки interchangeably for “because”?

All mean “because,” but style differs:

бо — short, very common in speech, slightly more informal.
тому що — neutral and widely used.
оскільки — more formal/bookish.
Your sentence could be: Учора я викликав майстра, тому що пральна машина зупинилася.
A comma is required before these conjunctions.

Is учора the same as вчора?

Yes, both mean “yesterday” and are standard. Вчора is more common in everyday use; учора is also correct and sometimes chosen for euphony or style. At the start of a sentence, either is fine: Учора/Вчора я…

Can I change the word order?

Yes, Ukrainian word order is flexible. Examples (all natural):

Я вчора викликав майстра, бо пральна машина зупинилася.
Вчора я викликав майстра, бо зупинилася пральна машина. (focus on “yesterday”)
Пральна машина зупинилася, тому я викликав майстра. (states the reason first)
Meaning stays the same; emphasis shifts slightly.

Could I say пральна машина зламалася/поламалася instead of зупинилася?

You can, but nuance changes:

зупинилася = “stopped (mid-cycle/at some point)” — may or may not be a breakdown.
зламалася/поламалася = “broke down” — stronger, clearly indicates a fault.
Choose based on what you want to convey.

Do I need the comma before бо?

Yes. Ukrainian normally uses a comma before causal conjunctions like бо, тому що, оскільки:

…викликав майстра, бо пральна машина зупинилася.

Can I drop я and just say Викликав майстра?

You can, but in the past tense it’s often unclear who did it, because викликав fits any masculine singular subject (I/he). Ukrainian usually keeps the pronoun in the past to make the subject explicit: Я викликав…

Is майстер gender-neutral? What if the technician is a woman?

In service contexts, майстер is commonly used regardless of the person’s actual gender. There is a feminine form майстриня, which you may see, but майстер remains the default job title in many settings. You can also use технік (“technician”) or be specific: майстер з ремонту пральних машин.