Kun me mennään autolla mökille, mä tarkistan turvavyöt ja sä huolehdit siitä, että kaikki on mielestäsi kunnossa.

Me mennään is spoken colloquial Finnish for standard me menemme (“we go / we are going”).

In everyday speech, especially in southern/urban Finnish, people very often use:

• me + passive form: me mennään, me tullaan, me syödään
  instead of
• me + 1st person plural: me menemme, me tulemme, me syömme

Both mean the same thing; me menemme just sounds more formal or written. In casual conversation, me mennään is much more natural.

Yes, in standard grammar mennään is the present passive (“people go / one goes / it is gone”).

However, in colloquial Finnish that same form is also used as we:

• Standard written:
  Me menemme autolla mökille.
• Colloquial spoken:
  Me mennään autolla mökille.

You recognize “real” passive vs colloquial “we” mainly from context and from whether there is an explicit subject:

• Passive without subject: Mennään autolla mökille. → “Let’s go to the cottage by car.” / “We’ll go…”
• Colloquial “we”: Me mennään autolla mökille. → clearly “we go”.

So the form is the same, but the presence of me makes it clearly “we” in spoken Finnish.

Autolla is the adessive case and here it means “by car / with a car” (instrumental use).

• auto = car
• autolla = “with a car / by car” (how you travel)

In Finnish, the adessive (-lla / -llä) is very often used for means of transport:

• mennä autolla = go by car
• mennä bussilla = go by bus
• mennä junalla = go by train
• mennä pyörällä = go by bike

Compare:

• autossa (inessive -ssa) = in the car (location inside)
• autolla (adessive -lla) = by car (as a means of transport)

So mennään autolla is “we go by car”.

All three forms exist, but they have different meanings:

• mökille = allative (“to the cottage (as a place / property)”)
• mökkiin (actually mökkiin) = illative (“into the cottage building / inside”)
• mökillä = adessive (“at the cottage / on the cottage property”)

In this sentence:

• mennään autolla mökille = “(we) go to the cottage (as a destination: the cottage and its yard etc.)”

If you said:

• mennään autolla mökkiin, it would focus on going inside the building itself.
• ollaan mökillä = “we are at the cottage”.

For going to someone’s house or cottage as a general place, Finnish very often uses the -lle form:

• mennään kaverille = we go to a friend’s (place)
• mennään mökille = we go to the cottage

Mä and sä are the most common spoken colloquial forms of minä (I) and sinä (you, singular).

So in the sentence:

• mä tarkistan = minä tarkistan (“I check”)
• sä huolehdit = sinä huolehdit (“you take care / you make sure”)

They mean exactly the same; minä/sinä are standard/written or more formal, mä/sä are everyday, casual spoken Finnish.

Turvavyöt is nominative plural (“seat belts”) and here it’s a total object: you check all the seat belts fully.

• turvavyö = a seat belt
• turvavyöt = seat belts (plural, whole seat belts as a complete set)

With a verb like tarkistaa (“to check”), if you are checking the object completely / as a whole thing, Finnish usually uses the nominative (or genitive) object, not partitive:

• Mä tarkistan turvavyöt. = I check (all) the seat belts.
• Mä tarkistan turvavöitä. would suggest an ongoing / partial checking (strange here).

Turvavyön (genitive singular) would be “the seat belt (one belt)”:

• Mä tarkistan turvavyön. = I check the (one) seat belt.

In the original sentence, the idea is “I check the seat belts (all of them)”, so turvavyöt fits best.

The verb is huolehtia. One common pattern is:

• huolehtia siitä, että + clause
  = “to see to it that / to take care that / to make sure that…”

Literally:

• sinä huolehdit = you take care / you see to it
• siitä = about/from that
• että kaikki on mielestäsi kunnossa = that everything is (in your opinion) OK

So:

sä huolehdit siitä, että kaikki on mielestäsi kunnossa
“you make sure that everything is OK in your opinion”

Why the siitä?
In Finnish, it’s very common to have a pronoun (se → siitä) before an että-clause that explains what “that” refers to. It’s a bit like saying in English: “You take care of it that everything is OK.” In English we usually drop of it, but Finnish keeps siitä.

Without siitä, the sentence is still understandable (sä huolehdit, että…), but huolehtia siitä, että… is the most natural idiom here.

In Finnish, että introduces a subordinate clause (a “that”-clause). The rule in standard writing is that a comma is placed before että:

• …sä huolehdit siitä, että kaikki on mielestäsi kunnossa.

So the main clause is:

• sä huolehdit siitä

and the subordinate clause is:

• että kaikki on mielestäsi kunnossa

This is different from English, where we don’t usually put a comma before “that” (“you make sure that everything is OK”). In Finnish punctuation, the comma is required here.

Mielestäsi means “in your opinion” or literally “from your mind”.

It is built from:

• mieli = mind, opinion
• mielestä = from (your) mind / in (someone’s) opinion
  
  • -stä = elative case “out of / from”
• mielestäsi = mielestä + -si (2nd person singular possessive suffix “your”)

So:

• mielestäni = in my opinion
• mielestäsi = in your opinion
• hänen mielestään = in his/her opinion
• meidän mielestämme = in our opinion
• etc.

In the sentence:

• kaikki on mielestäsi kunnossa
  = “everything is OK in your opinion.”

Kunnossa comes from:

• kunto = condition, shape
• kunnossa = “in (good) condition, in order, OK”

Grammatically, kunnossa is the inessive form of kunto (“in condition”), but it functions almost like a fixed expression:

• olla kunnossa = to be OK / to be in good order / to be in (good) condition

Examples:

• Auto on kunnossa. = The car is OK / in good shape.
• Kaikki on kunnossa. = Everything is fine / in order.

So:

• että kaikki on mielestäsi kunnossa
  = “that everything is OK in your opinion / as you see it.”

A more standard/written version (though still natural) would be:

Kun menemme autolla mökille, minä tarkistan turvavyöt ja sinä huolehdit siitä, että kaikki on mielestäsi kunnossa.

Changes:

• me mennään → menemme
• mä → minä
• sä → sinä

Everything else can stay the same; the grammar is already standard, just with colloquial pronouns and verb forms in the original.

You could also drop the subject pronouns in neutral written style, because the verb endings already show the person:

Kun menemme autolla mökille, tarkistan turvavyöt ja huolehdit siitä, että kaikki on mielestäsi kunnossa.

This is fully standard written Finnish.

The given order is the most neutral:

• että kaikki on mielestäsi kunnossa

Possible variants:

• että mielestäsi kaikki on kunnossa – also correct, with a bit more emphasis on your opinion.
• että kaikki mielestäsi on kunnossa – possible but feels slightly more marked/emphatic.

Word order in Finnish is relatively flexible, but neutral focus is typically:

• että + subject (kaikki) + verb (on) + adverbial (mielestäsi kunnossa)

Changing the order can shift emphasis, but all of these are grammatically possible. In everyday speech, että kaikki on mielestäsi kunnossa is the most straightforward and usual.