Olin juuri kuvaamassa, kun metro saapui.

Kuvaamassa is the third infinitive in the inessive case (-ssa/-ssä). With olla (to be), it means being in the middle of doing something — a natural way to express an English-like progressive.

• Olin kuvaamassa = I was (engaged in) filming/photographing.
• Related forms:
  • kuvaamaan (illative) = to go (in order) to film.
  • kuvaamasta (elative) = from (having been) filming.

Juuri means just/right then/exactly. It narrows the time to the very moment of the other event.

• Olin juuri kuvaamassa ≈ I was just then in the middle of filming.
• Juuri kun metro saapui… = Just as the metro arrived…
• Compare: vasta = only/only just (often implies you had barely started): Olin vasta kuvaamassa.

Finnish usually drops subject pronouns because the verb ending shows the person. Olin already tells us it’s first person singular. You can include Minä for emphasis or contrast:

• Minä olin juuri kuvaamassa… = It was me (not someone else) who was filming…

Yes. Finnish uses a comma to separate a subordinate clause from the main clause. When the kun-clause follows, you put a comma before kun:

• Olin juuri kuvaamassa, kun metro saapui. If the kun-clause comes first, you put the comma at its end:
• Kun metro saapui, olin juuri kuvaamassa.

Both can translate as arrived/came.

• Saapua is more formal/official or precise about arrival at a destination.
• Tulla is the everyday general verb to come/arrive. In casual speech many would say …kun metro tuli; …kun metro saapui is slightly more formal.

As the subject of an affirmative clause, metro is in nominative.

• metron (genitive) = the metro’s; or as an object form in some contexts, not as a subject here.
• metroa (partitive) = partitive; not used for a definite subject here.
• metrossa = in the metro (location), a different meaning altogether.

Yes. Both orders are correct. The difference is in focus:

• Olin juuri kuvaamassa, kun metro saapui. focuses on what you were doing when the arrival happened.
• Juuri kun metro saapui, olin kuvaamassa. highlights the exactness of the arrival moment; the filming is backgrounded.

Add -ma/-mä to the strong verb stem and then the case ending -ssa/-ssä. Examples:

• lukea → lukemassa (I was reading: Olin lukemassa)
• syödä → syömässä (I was eating: Olin syömässä)
• tehdä → tekemässä (I was doing/making: Olin tekemässä)
• katsoa → katsomassa (I was watching: Olin katsomassa)
• juosta → juoksemassa (I was running: Olin juoksemassa)
• käydä → käymässä (I was visiting/stopping by: Olin käymässä)

Yes, but it means something else. Olin juuri kuvannut is pluperfect: I had just filmed (i.e., I had just finished filming). Your original sentence says you were in the middle of filming when the metro arrived.

• Ongoing, interrupted: Olin juuri kuvaamassa, kun…
• Just completed: Olin juuri kuvannut, kun… You could also say: Olin juuri ollut kuvaamassa = I had just been filming (earlier), which is again about a finished activity before the arrival.

Both events are in the past, so saapui (past) matches olin (past). Other options change the timeline:

• Kun metro oli saapunut, olin kuvaamassa. = The arrival happened before you were filming (odd for the intended meaning).
• Kun metro oli saapumassa, olin kuvaamassa. = The metro was in the process of arriving while you were filming.
• Present (saapuu) would be narrative/historic present, a stylistic choice, not typical in neutral writing here.

No. Here -ssa is the inessive of the verbal noun and signals being inside the activity, not a physical location. You can combine both:

• Olin asemalla kuvaamassa = I was at the station filming.

Both, depending on context. If you want to be explicit:

• valokuvata = to photograph (still photos)
• filmata = to film/shoot video So Olin kuvaamassa can mean either photographing or filming; context clarifies.

A common colloquial rendering is:

• Mä olin just kuvaamas, ku metro tuli. Notes:
• mä for minä
• just for juuri
• ku for kun
• final -ssa often reduces to -s in speech (kuvaamas)
• tuli is the everyday verb instead of saapui