Suostuin uuteen suunnitelmaan.

Suostuin is the 1st person singular past tense form of the verb suostua.

• suostua = to agree, consent, go along with
• suostuin = I agreed / I consented

The ending -in is the usual marker for I in the past tense:

• minä suostuin = I agreed

In this sentence, minä is omitted because Finnish usually does not need the subject pronoun when the verb already shows the person.

Because Finnish often leaves out subject pronouns when they are clear from the verb ending.

So:

• Suostuin uuteen suunnitelmaan.
• Minä suostuin uuteen suunnitelmaan.

both mean the same thing, but the first is more natural in many contexts.

The ending -in in suostuin already tells you the subject is I.

Because the verb suostua requires its complement in the illative case.

So the basic pattern is:

• suostua johonkin = to agree to something

Here, johonkin is the clue word: it means into / to something, and it points to the illative.

That is why:

• uusi suunnitelma = a new plan
• uuteen suunnitelmaan = to the new plan

So the sentence uses the form required by the verb.

Suunnitelmaan is in the illative singular.

The illative often answers the question into what? / to what?

From the noun:

• suunnitelma = plan

to the illative:

• suunnitelmaan = into/to the plan

With suostua, this case does not necessarily describe physical movement. It is just the case that the verb governs.

So here it means something like:

• agree to a plan
• consent to a plan

In Finnish, adjectives usually match the noun they describe in case and number.

So if the noun is in the illative singular, the adjective must also be in the illative singular.

That is why:

• uusi suunnitelma = a new plan
• uuteen suunnitelmaan = to a new plan / to the new plan

Both words change together:

• uusi → uuteen
• suunnitelma → suunnitelmaan

This agreement is very important in Finnish.

They are both illative singular forms, but they are formed a little differently because the words belong to different noun/adjective patterns.

1. suunnitelma → suunnitelmaan

Many words ending in -a/-ä form the illative singular by adding a long vowel plus -n:

• talo → taloon
• huone → huoneeseen
• suunnitelma → suunnitelmaan

So here:

• stem: suunnitelma-
• illative ending: roughly -an
• result: suunnitelmaan

2. uusi → uuteen

This is a different word type. The stem changes:

• uusi
• stem: uute-
• illative: uuteen

This same pattern appears in words like:

• pieni → pieneen
• kaunis → kauniiseen (different but related kind of stem change)

So the forms may look irregular at first, but they follow common Finnish patterns.

Not always exactly. Suostua often has the sense of:

• agree to
• consent to
• be willing to accept
• go along with

It is often used when someone accepts a proposal, request, arrangement, or idea.

So Suostuin uuteen suunnitelmaan is close to:

• I agreed to the new plan
• I accepted the new plan
• I went along with the new plan

Depending on context, it can sound slightly more like consenting or accepting than simply sharing the same opinion.

These are related, but not the same.

suostua johonkin

= to agree to something, to consent, to accept a proposal

• Suostuin uuteen suunnitelmaan.
• I agreed to the new plan.

olla samaa mieltä

= to agree with someone / in opinion

• Olen samaa mieltä.
• I agree. / I am of the same opinion.

So if you mean I accepted the plan, suostua is a good choice.
If you mean I share your opinion, olla samaa mieltä is often better.

Yes. That can be a good translation in some contexts.

The exact English wording depends on tone and situation:

• I agreed to the new plan = most neutral
• I accepted the new plan = emphasizes acceptance
• I consented to the new plan = more formal
• I went along with the new plan = more conversational, sometimes suggests reluctance

So suostuin can cover several nearby English ideas.

Finnish usually does not mark a/the directly.

So uuteen suunnitelmaan could mean either:

• to a new plan
• to the new plan

The context tells you which one is meant.

That is normal in Finnish. Articles like a and the simply do not exist as separate words.

The dictionary forms are:

• suostuin → suostua
• uuteen → uusi
• suunnitelmaan → suunnitelma

So if you want to look them up, those are the forms you would search for.

Not completely. Finnish word order is fairly flexible, though not random.

The neutral order here is:

• Suostuin uuteen suunnitelmaan.

But other orders are possible if you want to emphasize something:

• Uuteen suunnitelmaan suostuin.
  This puts more focus on the new plan.

Still, the original version is the most natural neutral sentence.

Because the cases show the grammatical roles, Finnish can move words around more freely than English.

No, that would be incorrect with this verb.

The verb suostua requires the illative:

• suostua suunnitelmaan
• suostua ehdotukseen
• suostua pyyntöön

So you need:

• Suostuin uuteen suunnitelmaan.

Not:

• Suostuin uuden suunnitelman.

The form uuden suunnitelman is not the case this verb needs.

Here are some useful patterns:

• Suostuin ehdotukseen. = I agreed to the proposal.
• Hän suostui pyyntöön. = He/She agreed to the request.
• Emme suostuneet muutokseen. = We did not agree to the change.
• Suostutko tähän? = Do you agree to this?

These all show the same structure:

• suostua + illative

So the sentence Suostuin uuteen suunnitelmaan fits a very common pattern.