Otan mukaan kaiken, mitä tarvitsen.

It is an idiomatic combination meaning I take along / I bring with me.

• otan = I take
• mukaan adds the idea of along, with me, with us, with someone

So Finnish often expresses take with you as ottaa mukaan.

Because the verb already shows the subject.

• otan ends in -n, which marks first person singular: I take
• tarvitsen also ends in -n: I need

Finnish often leaves out subject pronouns when they are clear from the verb ending. You would add minä only for emphasis or contrast.

Because here the word means everything, and it is the object of the verb otan.

In Finnish, kaikki and kaiken are not interchangeable:

• kaikki often means all / everyone / all of them
• kaiken means everything / all of it in this kind of object position

So:

• Otan mukaan kaiken = I’m taking everything with me
• Otan mukaan kaikki would more naturally sound like I’m taking everyone / all of them with me

It is the genitive-looking total object form of kaikki.

In affirmative sentences, a complete object is often marked this way in Finnish. The idea here is that the speaker is taking the whole set of needed things, not just some of them.

That is why you get:

• kaiken = everything as a complete object

not:

• kaikkea = partitive, which would suggest an incomplete or non-total object

Grammatically, it is singular, even though its meaning can cover many items.

Finnish often treats everything as a single totality. So kaiken is singular in form, but semantically it can refer to many separate things.

This is similar to how English everything is grammatically singular even though it includes many things.

Because after kaiken (everything), Finnish normally uses a relative pronoun from the mikä family, not the joka family.

So the pattern is:

• kaiken, mitä... = everything that...

This is a very common structure in Finnish, and it is worth learning as a chunk.

Mitä is the partitive singular form of mikä.

In this sentence it introduces the relative clause mitä tarvitsen = that I need / what I need.

For learners, the easiest thing is often to remember the whole pattern:

• kaiken, mitä tarvitsen
• everything that I need

You will see this same structure in many similar sentences.

Because ottaa mukaan is the normal fixed expression for take along.

Although mukaan and mukana are related, they are not used the same way here:

• ottaa mukaan = take along
• olla mukana = be along / be included / be with

So in this sentence, mukaan is the correct form because it belongs to the verb expression ottaa mukaan.

The dictionary form is tarvita, which means to need.

Here:

• tarvitsen = I need

So mitä tarvitsen means what I need or that I need.

Because mitä tarvitsen is a relative clause, and standard Finnish puts a comma before relative clauses.

So:

• kaiken, mitä tarvitsen

This is different from English, where everything that I need usually has no comma.

Yes, Finnish word order is somewhat flexible, but this version is the most neutral and natural:

• Otan mukaan kaiken, mitä tarvitsen.

It keeps the common verb phrase otan mukaan together and then adds the object plus its relative clause.

Other word orders may be possible, but they can sound more marked, more literary, or place different emphasis on parts of the sentence.