Yksi nappi puuttuu takistani, joten vien sen ompelijalle.

Puuttua means to be missing or to be lacking.

In Finnish, the thing that is missing is usually the grammatical subject. So:

• Yksi nappi puuttuu takistani
  = literally something like One button is missing from my coat

This is different from how English sometimes phrases it, such as My coat is missing a button. Finnish normally builds it around the missing thing itself.

Because yksi nappi is the subject of the sentence, so it is in the nominative singular.

• yksi = one
• nappi = button

If you used yhtä nappia, that would be partitive, and it would not fit this structure with puuttuu in a normal neutral sentence like this.

Takistani breaks down like this:

• takki = coat
• takista = from the coat
• takistani = from my coat

So Finnish packs several meanings into one word:

• the noun
• the case ending
• the possessive ending

Here the case is elative (-sta / -stä), which often means out of / from.

Because Finnish often shows possession with a possessive suffix attached to the noun.

Here:

• -ni = my

So:

• takistani = from my coat

You can also say minun takistani, but in many contexts the suffix alone is enough, and that is very normal Finnish.

This happens because Finnish nouns change form when case endings are added, and many words also go through consonant gradation.

With takki, the kk becomes k in many inflected forms:

• takki = coat
• takin
• takissa
• takista

This kind of stem change is very common in Finnish and is something learners just gradually get used to.

Joten means so, therefore, or thus.

It connects the first clause to the consequence:

• a button is missing
• therefore / so I take it to the tailor

It is a bit more formal or written-sounding than the most casual ways of linking clauses, but it is completely normal.

Yes, but Finnish does not have a separate future tense the way English does.

So the present tense often covers both:

• what is happening now
• what will happen soon
• what someone intends to do

So vien can mean:

• I am taking
• I take
• I will take

The context tells you which meaning is intended.

Because viedä means to take/bring something somewhere, while mennä means simply to go.

• vien sen ompelijalle = I take it to the tailor
• menen ompelijalle = I go to the tailor

In this sentence, the speaker is talking about taking an object somewhere, so viedä is the right verb.

Sen refers to the coat, not the button.

Even though the first clause talks about the missing button, the second clause says the speaker will take it to the tailor. Logically, that means the coat, because that is the item that needs repairing.

So the reference is understood from the situation, not just from the nearest noun.

Because here sen is the normal object form for a complete, whole object in an affirmative sentence.

With viedä, the action is directed at a complete item:

• vien sen ompelijalle = I take it to the tailor

Using sitä would give a more partitive sense and would not sound as natural here.

A simple learner-friendly way to think about it is:

• sen = the whole item
• sitä = some of it / not viewed as a complete whole / ongoing or indefinite in certain contexts

Because -lle is the allative ending, which often means to or onto, and it is very commonly used for movement to a person.

So:

• ompelija = tailor / sewing professional
• ompelijalle = to the tailor

This is very typical Finnish:

• mennä lääkärille = go to the doctor
• viedä auto mekaanikolle = take the car to the mechanic
• viedä takki ompelijalle = take the coat to the tailor

Yes. Finnish word order is fairly flexible.

For example, this would also be natural:

• Takistani puuttuu yksi nappi, joten vien sen ompelijalle.

The difference is mostly about focus:

• Yksi nappi puuttuu takistani puts the spotlight on the missing button.
• Takistani puuttuu yksi nappi starts with the coat and then tells what is missing from it.

Both are grammatically fine.

Not always exactly.

Ompelija literally means a person who sews, so depending on context it can mean something like:

• tailor
• seamstress / seamster
• dressmaker
• sewing professional

If you want a more specific word for a tailor, Finnish also has räätäli. But in a sentence about taking a coat to someone who can fix it, ompelija works well.