Käyn sovituskopissa ennen kuin päätän, ostanko uuden takin vai en.

Both can be possible in different contexts, but they are not exactly the same.

• käydä + place in -ssa/-ssä often means to go somewhere briefly / to visit / to stop by
• mennä + place in -Vn / -seen / -iin focuses more directly on going into a place

So käyn sovituskopissa is a very natural way to say that you go to the fitting room and spend a moment there, for example to try something on.

In other words:

• käyn sovituskopissa = I’ll go/use the fitting room
• menen sovituskoppiin = I go into the fitting room

The sentence uses käydä because the fitting room is treated as a place you visit briefly.

Käyn is the 1st person singular present tense of käydä.

In this sentence, it does not literally mean only I walk or I go. With places, käydä often means:

• go to
• visit
• pop into
• use briefly

So here it means something like I go into / use the fitting room before deciding...

The ending -ssa is the inessive case, which usually means in.

• sovituskoppi = fitting room / changing booth
• sovituskopissa = in the fitting room

But this is especially important here because käydä often takes a place in:

• inessive (-ssa/-ssä) for indoor places
• adessive (-lla/-llä) for some other locations

So the form is not chosen only because of the English idea in. It is also chosen because the verb käydä commonly works that way.

Ennen kuin means before when it introduces a full clause.

Here it is followed by a verb:

• ennen kuin päätän = before I decide

Finnish uses:

• ennen on its own before a noun phrase
• ennen kuin before a clause with a finite verb

For example:

• ennen päätöstä = before the decision
• ennen kuin päätän = before I decide

So in this sentence, kuin is needed because what follows is a full clause.

Because ostanko uuden takin vai en is a subordinate clause, and Finnish normally separates subordinate clauses with commas.

The structure is:

• Käyn sovituskopissa ennen kuin päätän,
• ostanko uuden takin vai en.

That second part is an embedded question: whether I will buy a new coat or not.

Finnish punctuation uses commas more regularly with subordinate clauses than English sometimes does, so the comma is completely normal here.

Ostanko comes from ostan + -ko.

Breakdown:

• ostaa = to buy
• ostan = I buy
• ostanko = do I buy? / whether I buy

The ending -ko/-kö is the Finnish yes/no question marker.

So ostanko literally looks like buy-I-question, and here it means whether I buy because it is part of an embedded question, not a direct question.

Because Finnish often forms embedded yes/no questions with -ko/-kö attached directly to the verb.

In English, you say:

• I decide whether I buy it
• I don’t know if I should go

In Finnish, the equivalent often looks like:

• päätän, ostanko...
• en tiedä, menenkö...

So instead of a separate word like whether, Finnish usually puts the question marking on the verb itself.

Because uuden takin is the object of ostaa, and in this sentence it is a total object.

• uusi takki = a new coat (basic dictionary/nominative form)
• uuden takin = a new coat as a total object in the singular

With a verb like ostaa, when the action is seen as complete and the object is a whole thing, Finnish usually uses this genitive-looking accusative form in the singular.

So:

• ostan uuden takin = I buy a new coat

If you used the partitive uutta takkia, that would suggest a different meaning or aspect and would not be the normal choice here.

Because adjectives in Finnish usually agree with the noun they describe.

So if the noun changes case, the adjective changes too:

• uusi takki = nominative
• uuden takin = same case on both words

This agreement is very normal in Finnish. You do not leave the adjective in its basic form when the noun changes case.

Vai en means or not.

So:

• ostanko uuden takin vai en = whether I buy a new coat or not

This is a common Finnish pattern in embedded yes/no questions.

You can think of it as making both possibilities explicit:

• I buy it
• I do not buy it

Sometimes Finnish can leave out vai en if the meaning is already clear, but including it makes the contrast very clear and often sounds natural.

Because vai is used in questions and alternatives, while tai is the more general word for or in statements.

Here the sentence contains an embedded question:

• ostanko ... vai en = whether I buy ... or not

So vai is the correct choice.

A useful rule of thumb:

• vai = used with alternatives in questions
• tai = used for ordinary or in statements

Because Finnish usually does not need a separate subject pronoun when the verb ending already shows the person.

• päätän = I decide
• ostanko = do I buy / whether I buy

The I is already built into the verb form, so minä is unnecessary unless you want extra emphasis or contrast.

Yes. Finnish often uses the present tense for future meaning when the context makes it clear.

Here:

• päätän = I decide
• ostanko = whether I buy

Even though they are present forms, the meaning is naturally future-oriented: first you go to the fitting room, then you decide, then maybe you buy the coat.

This is completely normal Finnish. There is no separate future tense that would be required here.