Learn Hardcore Finnish: Yksi tavoitteeni on puhua suomea samalla, kun selitän videolla, mitä teen puutarhassa. - One of my goals is to speak Finnish while I explain in a video what I am doing in the garden.

Yksi tavoitteeni literally: one goal-of-mine → “one of my goals” / “one goal of mine”.
Minun tavoite is colloquial / non‑standard; in correct written Finnish you would say minun tavoitteeni (“my goal/goals”).

For “one of my goals”, very common alternatives are:

Yksi tavoitteistani on…
Yksi minun tavoitteistani on…

Your sentence’s Yksi tavoitteeni on… is grammatical and natural; it sounds slightly more written or “tidy” than Yksi minun tavoite.

What exactly does the ending -ni in tavoitteeni mean, and is it singular or plural?

The ending -ni is the 1st person singular possessive suffix, meaning my.

tavoite = goal
tavoitteeni = my goal / my goals

With many nouns, especially those ending in -e, the form with -ni is ambiguous in number:

huoneeni = my room / my rooms
tavoitteeni = my goal / my goals

Context tells you which one is meant.

In Yksi tavoitteeni on…, we interpret tavoitteeni as my goals, so the whole phrase is “one (of) my goals is…”. The word yksi (“one”) is what tells us we are talking about several possible goals.

Could I also say Yksi minun tavoitteistani on puhua suomea…? How does that differ from Yksi tavoitteeni on…?

Yes, you can say:

Yksi minun tavoitteistani on puhua suomea…

Differences:

Yksi tavoitteeni on…
- Slightly more compact, common in written language.
- Still understood as “one of my goals”.
Yksi tavoitteistani on… or Yksi minun tavoitteistani on…
- Literally “one from my goals”.
- Makes the “one of many” idea completely explicit.
- Very typical and completely natural in both spoken and written Finnish.

Meaning-wise, in this context they are essentially the same. Using minun adds a bit of emphasis on my: “one of my goals”.

Why is puhua in the basic infinitive form (puhua) after on, not conjugated like puhun?

Finnish often uses the 1st infinitive (the dictionary form) to express “to do something” after olla (to be) with nouns like goal, plan, wish.

Pattern:

Tavoitteeni on puhua suomea.
My goal is to speak Finnish.

Structure:

tavoitteeni – the subject complement (my goal)
on – is
puhua suomea – infinitive phrase, what the goal is

Using a finite verb instead would be odd here:

✗ Yksi tavoitteeni on puhun suomea – ungrammatical.
Yksi tavoitteeni on, että puhun suomea… – grammatical but feels more like “one of my goals is that I (actually) speak Finnish”, i.e. more like a statement about reality than an abstract aim to do X.

So [noun] + on + infinitive is the normal way to say “[noun] is to do X” in Finnish.

Why is it puhua suomea and not puhua suomi?

After puhua (to speak), the language name is normally in the partitive case:

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

Reasons:

puhua + partitive is the idiomatic pattern for “to speak a language (in general)”.
Nominative suomi on its own is usually the name of the language or country (“Finnish”, “Finland”), not the object of puhua.

So:

Haluan puhua suomea. – I want to speak Finnish.
✗ Haluan puhua suomi. – sounds wrong to a native ear.

What is the difference between puhua suomea and puhua suomeksi? Could I use suomeksi here?

Both are possible, but they emphasize slightly different things:

puhua suomea – “to speak Finnish (as a language)”
- Focus: the activity of speaking that language.
puhua suomeksi – “to speak in Finnish”
- -ksi here means “in the role of / as / in the form of”.
- Often used when you focus on the language used as a medium for some other purpose:
  - Selitän tämän suomeksi. – I’ll explain this in Finnish.

In your sentence:

Yksi tavoitteeni on puhua suomea… – perfectly natural: one of my goals is to speak Finnish…
Yksi tavoitteeni on puhua suomeksi… – also possible, but puts a bit more focus on Finnish as the medium of the explanation. It would sound like “my goal is that I do the explaining in Finnish”.

Both would be understood; puhua suomea is the most neutral here.

What does samalla, kun mean exactly, and why is there a comma before kun?

Components:

samalla – literally “with the same [thing]”, but as an adverb it means “at the same time / simultaneously”.
kun – a conjunction meaning “when / as / while”.

Together:

samalla, kun selitän… ≈ “while I explain…” / “at the same time as I explain…”

About the comma:

In standard Finnish punctuation, you put a comma before a subordinate clause introduced by kun, when that clause comes after the main clause.
So: Yksi tavoitteeni on puhua suomea samalla, kun selitän…

You could also say:

Yksi tavoitteeni on, että puhun suomea samalla kun selitän…

There samalla kun selitän is a unit after että, and the comma is before että instead.

You will often see samalla kun written without a comma in more casual writing, but the norm in careful written Finnish is with the comma in this structure.

In selitän videolla, what does the ending -lla in videolla mean? Why not videossa?

videolla is adessive case (ending -lla / -llä), which often expresses:

location on / at something,
or a medium / channel / means.

Here:

selitän videolla ≈ “I explain on video” / “in a video (as a format)”.

Rough parallels:

taululla – on the board
paperilla – on paper
netissä – on the internet (here inessive is used idiomatically)
videolla – on video

If you said:

videossa (inessive, in the video), it would sound more like something is inside the content of a particular video file:
- Tämä kohta selitetään videossa. – This point is explained in the video.

Your sentence is about the medium you are using to explain (video), so videolla is the natural choice.

Why is it mitä teen and not mikä teen?

The difference is case:

mikä – nominative; used mainly when “what” is the subject.
mitä – partitive; used in many other roles, including as an object.

In mitä teen, what is the object of teen (“I do / I’m doing”), so the form is mitä.

Compare:

Mikä tämä on? – What is this? (subject: mikä)
Mitä teen? – What am I doing? (object: mitä)
Mitä teen puutarhassa? – What am I doing in the garden?

In your sentence, mitä teen puutarhassa is an embedded question (“what I am doing in the garden”), so we still use mitä, not mikä.

Why is the verb order mitä teen instead of something like “what I do”, as in English “what I do in the garden”?

Finnish embedded questions keep the same word order as direct questions:

Direct question: Mitä teen puutarhassa? – What am I doing in the garden?
Embedded: …mitä teen puutarhassa. – …what I am doing in the garden.

So the pattern is:

Question word (e.g. mitä)
Verb (teen)
Other elements (puutarhassa)

In English, embedded questions switch back to subject–verb order (“what I do”), but Finnish does not switch: it stays in “question order”.

You could add the pronoun for emphasis:

…mitä minä teen puutarhassa.

But even then, mitä teen (verb after the question word) is the core structure.

Why is the pronoun minä omitted in kun selitän videolla, mitä teen puutarhassa?

In Finnish, personal pronouns are usually dropped when the person is clear from the verb ending:

selitän – I explain
teen – I do / I make

Because these endings already show 1st person singular, you don’t need minä:

kun selitän videolla – when I explain on video
mitä teen puutarhassa – what I’m doing in the garden

You can include minä for emphasis or contrast:

kun minä selitän videolla – when I explain (as opposed to someone else)
mitä minä teen puutarhassa – what I am doing (emphasis on I)

But the neutral, everyday style drops the pronoun.

What does the ending -ssa in puutarhassa express, and how would the meaning change with other endings?

puutarhassa is inessive case (ending -ssa / -ssä), which usually means “in / inside”:

puutarha – a garden
puutarhassa – in the garden

Other related cases:

puutarhaan (illative, into the garden) – movement into
- Menen puutarhaan. – I’m going into the garden.
puutarhalla (adessive, at the garden / by the garden) – more like “at/around” the garden area.
puutarhasta (elative, out of / from the garden) – movement out of
- Tulen puutarhasta. – I’m coming from the garden.

In your sentence, you’re describing an activity happening in a place, so puutarhassa (“in the garden”) is the natural choice.

The English meaning is “one of my goals is…”, which talks about more than one goal. Why does Finnish use singular on instead of plural ovat in Yksi tavoitteeni on…?

In Finnish, the verb agrees with the grammatical head of the subject phrase.

Here the head is yksi (one), which is singular, not tavoitteeni:

yksi tavoitteeni – one of my goals
Verb: on (singular), not ovat (plural)

So:

Yksi tavoitteistani on… – One of my goals is…
✗ Yksi tavoitteistani ovat… – incorrect

Even though the real-world set “my goals” is plural, grammatically you are asserting something about one member of that set, so Finnish uses the singular verb.