Haluan selittää sinulle ongelmani lisäksi myös uuden idean, johon tarvitsen apuasi.

Because selittää normally takes the person receiving the explanation in the allative case:

• selittää jollekin = to explain something to someone

So:

• sinä = you
• sinulle = to you

A few contrasts:

• sinä = you (subject form)
• sinua = you (object form in some contexts)
• sinulle = to you

In this sentence, sinulle marks the listener/recipient of the explanation.

Because Finnish often adds possession directly to the noun with a possessive suffix.

• ongelma = problem
• ongelmani = my problem

The ending -ni means my.

So instead of using a separate word like English my, Finnish can attach it to the noun itself.

You can also say:

• minun ongelmani

That is also my problem, but here the shorter form ongelmani is enough and very natural.

They are similar, but not identical.

• lisäksi = in addition, besides
• myös = also, too

Together, ongelmani lisäksi myös uuden idean gives a stronger additive idea:

• not only my problem,
• but also a new idea

So it is not wrong or strange. It is a way of emphasizing that one more thing is being added.

In many contexts, one of them could be left out, but using both is perfectly natural.

Because uuden idean is the object of selittää.

In Finnish, when the object is seen as a whole, completed thing, a singular total object often looks like the genitive form:

• uusi idea = new idea (basic dictionary form / nominative)
• uuden idean = the new idea / a new idea as a complete object

Also, the adjective agrees with the noun:

• uusi
  • idea
• uuden
  • idean

So both words change together.

Because uutta ideaa would be the partitive.

The partitive object is often used when the action is:

• incomplete
• ongoing
• indefinite
• only affecting part of something

But here the speaker wants to explain a whole idea, not just some vague amount of idea. So uuden idean is the natural choice.

Very roughly:

• selittää uuden idean = explain a whole new idea
• selittää uutta ideaa = explain some new idea / be in the process of explaining a new idea

johon is a form of the relative pronoun joka.

• joka = who / which / that
• johon = into which / to which / for which, depending on context

In Finnish, a relative pronoun takes the case required by its role inside the relative clause.

Here the clause is:

• johon tarvitsen apuasi

This refers back to uuden idean.

The important point is that the verb phrase requires this case relationship, roughly like:

• tarvitsen apuasi johonkin = I need your help for something / with something

So when johonkin becomes a relative pronoun referring back to idean, it becomes johon.

Because tarvita usually takes a partitive object.

• tarvitsen apua = I need help

When you add the possessive suffix -si (your), you get:

• apuasi = your help

So:

• apu = help
• apua = help (partitive)
• apuasi = your help (partitive + possessive suffix)

By contrast, apusi is not the right form here, because that would not match the object pattern required by tarvita.

Because Finnish often leaves the possessive meaning inside the noun itself, using a suffix.

• ongelmani = my problem
• apuasi = your help

So separate pronouns are not necessary.

They can be added for emphasis:

• minun ongelmani
• sinun apuasi

But in this sentence, the suffixes already tell us whose problem and whose help are meant.

Because johon tarvitsen apuasi is a relative clause describing uuden idean.

Finnish normally puts a comma before subordinate clauses like this, including relative clauses.

So the comma is standard punctuation:

• uuden idean, johon tarvitsen apuasi

It works like English punctuation before a descriptive clause, although Finnish comma rules are a bit more regular and obligatory.

Finnish word order is fairly flexible, but this version is natural and clear.

The structure is roughly:

• Haluan = I want
• selittää = to explain
• sinulle = to you
• ongelmani lisäksi myös uuden idean = in addition to my problem, also a new idea
• johon tarvitsen apuasi = for which I need your help

This order keeps the sentence easy to follow:

1. what I want to do
2. who I want to do it to
3. what I want to explain
4. extra information about the new idea

Other word orders are possible, but they would change emphasis. Finnish often moves things around for focus, but this sentence is a good neutral choice.