Learn Hardcore Finnish: Päätin julkaista lyhyen videon, jossa puhun suomea kahvilassa. - I decided to publish a short video in which I speak Finnish in a café.

Päätin already means “I decided”.
You can add Minä päätin if you want to emphasize the subject (I as opposed to someone else), but normally just Päätin is enough and more natural.

Why is julkaista in the infinitive, and how does päätin julkaista work?

After päättää (to decide), Finnish uses the basic infinitive (1st infinitive) of the next verb:

päätin julkaista = I decided to publish
päätin mennä = I decided to go
päätin ostaa = I decided to buy

So:

julkaista is the basic dictionary form of the verb (to publish).
The structure is: [päätin] + [infinitive] = I decided to [do something].

You would not normally change julkaista to another form here; päätin julkaista is the standard construction.

What case is lyhyen videon, and why isn’t it just lyhyt video?

Lyhyen videon is in the genitive singular:

lyhyt video = short video (basic/nominative form)
lyhyen videon = short video (genitive: of the short video / the short video as a total object)

Both words change:

lyhyt → lyhyen (adjective)
video → videon (noun)

In this sentence, lyhyen videon is the object of the verb julkaista (to publish). A completed, definite object in many contexts appears in genitive in Finnish. So:

Päätin julkaista lyhyen videon.
= I decided to publish the/a short video (a complete, whole video)

If you said lyhyt video, that would normally be the subject form, not a typical object form in this context.

How do I know when to put the object in genitive (videon) vs nominative (video) vs partitive (videota)?

This is the Finnish object case system. Very simplified:

Genitive (videon) — “total” or complete object
- Often when the action is bounded/complete or you clearly refer to the whole thing.
- E.g. Julkaisen videon. = I will publish the (whole) video.
Partitive (videota) — “partial” or unbounded object
- When the action is ongoing, incomplete, repeated, or affects only part of something, or when it’s uncertain if it will be completed.
- E.g. Katson videota. = I’m watching (some of) a video / I’m in the process of watching a video.
Nominative (video) as object
- Appears mainly with plural objects and some special cases (like imperatives or with certain auxiliaries).
- Plural example: Julkaisen videot. = I will publish the videos.

In Päätin julkaista lyhyen videon, the idea is a definite, whole video that will be published completely, so genitive is natural: videon.

How can videon mean both “the video” and “a video” when there are no articles in Finnish?

Finnish has no articles (a / an / the). The form videon does not itself say whether it’s a video or the video. That meaning comes from context:

If both speaker and listener know which video is meant, you translate it as “the video”.
If it’s just any short video, you translate it as “a video”.

So lyhyen videon can be:

a short video
the short video

depending on the situation. The Finnish grammar stays the same.

What does jossa mean exactly, and how is it different from missä?

Both are related to the idea of “where / in which”, but they are used differently.

missä
- Question word: Missä olet? = Where are you?
- Can also be used as a kind of relative pronoun in colloquial speech, but this is less standard.
jossa
- A relative pronoun: “in which / where (in)”
- It refers back to a noun mentioned earlier.

In the sentence:

lyhyen videon, jossa puhun suomea kahvilassa
= a short video *in which I speak Finnish in a café*

Here jossa refers back to videon. Literally: the short video, in-which I speak Finnish...
In standard written Finnish, jossa is the correct choice here, not missä.

Why is there a comma before jossa?

Finnish uses a comma to separate the main clause from a subordinate clause.

Main clause: Päätin julkaista lyhyen videon
Relative clause: jossa puhun suomea kahvilassa

Relative clauses often start with joka / jota / jossa / johon, etc. In standard writing, there is usually a comma before these when they introduce a subordinate clause:

…, jossa puhun suomea kahvilassa.
…, joka on uusi. (…, which is new.)
…, että tulen huomenna. (…, that I will come tomorrow.)

So the comma is a normal punctuation rule in Finnish for such clauses.

Why is suomea in the partitive case, not just suomi?

Suomea is the partitive form of suomi (Finnish as a language).

The verb puhua (to speak) usually takes the partitive when you talk about a language:

puhua suomea = to speak Finnish
puhua englantia = to speak English
puhua ruotsia = to speak Swedish

Reasons (simplified):

Languages are treated somewhat like mass / uncountable nouns (like “water, sand”).
The partitive often expresses an indefinite amount or ongoing activity.

So puhun suomea = I speak/am speaking Finnish, and puhun suomi would be ungrammatical in this sense.

What case is kahvilassa, and how is it different from kahvilaan or kahvilasta?

These are local cases indicating location or movement.

kahvilassa = in the café
- Case: inessive (-ssa / -ssä) = inside / in
kahvilaan = into the café
- Case: illative (-an, -en, -in, etc.) = to / into (going in)
kahvilasta = out of/from the café
- Case: elative (-sta / -stä) = out of / from inside

In the sentence:

jossa puhun suomea kahvilassa
= in which I speak Finnish *in a café (location, not movement)*

So kahvilassa is correct because you are inside / at the café while speaking.

Is the word order jossa puhun suomea kahvilassa fixed, or can I move things around?

Finnish word order is more flexible than English, but there is still a neutral, most natural order.

Here, jossa puhun suomea kahvilassa is a very typical order:

Verb: puhun
Object / complement: suomea
Place: kahvilassa

You can move elements for emphasis, but it starts to sound marked or odd if you move them too much. For example:

jossa kahvilassa puhun suomea – sounds strange; it suggests a contrast like “in which café I speak Finnish”, which doesn’t match the rest well.
jossa puhun kahvilassa suomea – possible, but less neutral; the emphasis can slightly shift.

For everyday, neutral speech and writing, jossa puhun suomea kahvilassa is the best choice.

Could I say jossa minä puhun suomea? When do Finns actually use the pronoun minä?

Yes, jossa minä puhun suomea kahvilassa is grammatically correct.

But most of the time, Finnish omits personal pronouns because the verb ending already shows the person:

puhun = I speak
puhut = you speak
puhuu = he/she speaks

You normally add minä only when:

You want emphasis or contrast:
- Minä puhun suomea, mutta hän puhuu ruotsia.
  = I speak Finnish, but he/she speaks Swedish.
You’re clarifying who the subject is in a complex context.

In your sentence, there is no particular contrast, so jossa puhun suomea kahvilassa (without minä) is more natural.

Could I say videon, missä puhun suomea kahvilassa instead of videon, jossa puhun suomea kahvilassa?

In colloquial spoken Finnish, you will hear things like:

video, missä puhun suomea
(the video where I speak Finnish)

Here missä is used as a relative word instead of jossa.

However, in standard written Finnish (textbooks, official writing, careful writing):

jossa is the correct relative pronoun here.
missä is mainly a question word (where?) and is not preferred as a relative pronoun in formal style.

So:

Spoken / informal: videon, missä puhun suomea… – common, but informal.
Standard / written: videon, jossa puhun suomea kahvilassa – recommended in writing.