Learn Hardcore Finnish: Aion päivittää profiilin huomenna. - I am going to update the profile tomorrow.

So Aion päivittää profiilin huomenna is literally "I intend to update the profile tomorrow", often understood in English as "I’m going to update the profile tomorrow".

Finnish does not have a special future tense like English. Instead it usually uses:

the present tense:
- Päivitän profiilin huomenna. = I will update / I’m updating the profile tomorrow.
or verbs like aikoa (to intend), tulla (to come to), etc., to make the future more explicit:
- Aion päivittää profiilin huomenna. = I’m going to (I intend to) update the profile tomorrow.

Could you just say Päivitän profiilin huomenna instead? What’s the difference in nuance?

Yes, you can say:

Päivitän profiilin huomenna.
Aion päivittää profiilin huomenna.

Both are correct and can refer to the future.

Nuance:

Päivitän profiilin huomenna.
- Simple statement; sounds like a decision/fact.
- Very normal, especially in neutral or written language.
Aion päivittää profiilin huomenna.
- Emphasizes intention or plan.
- Similar to English “I’m going to update…” or “I intend to update…”.
- Often sounds a bit more deliberate: you have already decided to do it.

In many contexts they can be used almost interchangeably, with only a slight difference in feel.

Why is päivittää in the infinitive here and not päivitän?

The verb aikoa works a bit like English “to be going to” or “to intend to”. It is followed by a verb in the basic infinitive (1st infinitive) form:

aion + päivittää = I intend to update
aiot + syödä = you intend to eat
aikoo + opiskella = he/she intends to study

So the pattern is:

[aikoa, conjugated] + [main verb in basic form (infinitive)]

You don’t say “aion päivitän” for the same reason you don’t say in English “I intend I update”. You need “to update” → in Finnish: päivittää.

Why does profiilin end in -n instead of just profiili?

The -n ending here marks the object case (often called genitive/accusative in Finnish grammar).

profiili = a profile (dictionary form, nominative)
profiilin = the profile (as a total object of the verb)

In Aion päivittää profiilin huomenna the idea is that you will fully update the whole profile. For this kind of complete, bounded action, Finnish usually puts the object in the genitive/accusative:

Luetko kirjan? = Will you read the (whole) book?
Syön omenan. = I’ll eat the apple (completely).
Päivitän profiilin. = I’ll update the profile (as a complete task).

So profiilin is the “normal” total object form of profiili in this sentence.

What’s the difference between profiilin and profiilia?

Both are forms of profiili, but they express different things as objects:

profiilin (genitive/accusative):
- total object – action is seen as complete/bounded.
- Example:
  - Aion päivittää profiilin huomenna.
    = I’m going to update the profile (as a complete task).
profiilia (partitive):
- partitive object – action is ongoing, incomplete, or unbounded, or you’re only dealing with part of it.
- Example:
  - Päivitän profiilia huomenna.
    = I’ll be updating the profile tomorrow (working on it, not necessarily finishing).

In negative sentences, Finnish normally uses the partitive for the object:

En päivitä profiilia huomenna.
= I will not update (work on) the profile tomorrow.

How would you say “my profile” here? Do you need a separate word for my?

You have two main options:

Possessive suffix (more standard / written):
- Profiilini = my profile
- Aion päivittää profiilini huomenna.
  = I intend to update my profile tomorrow.
Here the -ni suffix means my. You don’t need a separate word like “my”.
Spoken-style “my” word + normal noun (common in speech):
- mun profiili = my profile (informal; mun = my)
- Aion päivittää mun profiilin huomenna.

Both are common. In formal writing, profiilini is preferred; in everyday speech, mun profiili / mun profiilin is very typical.

Where can huomenna go in the sentence? Is the word order flexible?

Yes, Finnish word order is relatively flexible. All of these are possible:

Aion päivittää profiilin huomenna. (neutral)
Aion huomenna päivittää profiilin.
Huomenna aion päivittää profiilin.
Huomenna päivitän profiilin. (without aion)

Nuance:

First position (Huomenna...) often emphasizes the time: tomorrow (and not today).
Placing huomenna right after the verb (Aion huomenna päivittää...) is also fine and common.

The default, neutral version is the one you originally gave:
Aion päivittää profiilin huomenna.

Do you need to say minä as in Minä aion päivittää profiilin huomenna, or is Aion päivittää profiilin huomenna enough?

You do not need to say minä. The verb ending -n in aion already shows that the subject is I.

Aion päivittää profiilin huomenna. = I’m going to update the profile tomorrow.
Minä aion päivittää profiilin huomenna. = I am going to update the profile tomorrow (with emphasis on “I”).

Use minä when:

you want to contrast with someone else:
- Minä aion päivittää profiilin, mutta sinä et.
or you really want to stress the subject.

In everyday neutral sentences, Finns often drop the pronoun.

How do you pronounce aion and päivittää?

Approximate pronunciations:

aion
- syllables: ai-on
- ai like English eye
- o like in or but shorter
- n as in no
- roughly: “eye-on” (said smoothly as two syllables).
päivittää
- syllables: päi-vit-tää
- ä like the a in cat (but a bit more front)
- äi similar to eye but with ä at the start
- v as in very
- tt is a long / double t – hold the t a bit longer
- ää is a long ä – hold the sound: ää
- stress is always on the first syllable: PÄI-vit-tää.

So very roughly: “PAI-vit-tae”, with a longer t and a long ä at the end.

Is aion (from aikoa) formal, informal, or neutral? Are there more casual alternatives?

Aion is neutral. You can use it in both spoken and written Finnish.

More casual or alternative ways to express a similar idea:

Ajattelin päivittää profiilin huomenna.
= I was thinking of updating the profile tomorrow.
Meinaan päivittää profiilin huomenna. (very colloquial)
= I’m gonna update the profile tomorrow.
Päivitän profiilin huomenna.
= I’ll update the profile tomorrow (simple present with future meaning).

So aion is perfectly natural and not particularly formal; it just highlights intention a bit more explicitly.

Why isn’t there a separate word for “to” (as in “to update”) before päivittää?

In Finnish, the idea of English “to” in “to do, to see, to update” is baked into the infinitive ending:

päivittää = to update
syödä = to eat
lukea = to read
opiskella = to study

So after verbs like aikoa, you just put the main verb in its basic infinitive form; no extra word like English “to” is used:

aion päivittää = I intend to update
aion syödä = I intend to eat

How would you say “I am not going to update the profile tomorrow” in Finnish?

A natural translation is:

En aio päivittää profiilia huomenna.

Breakdown:

En = I do not (1st person singular negative verb)
aio = intend (negative form of aion)
päivittää = to update
profiilia = the profile (in partitive because the sentence is negative)
huomenna = tomorrow

Notice the change profiilin → profiilia.
In negative sentences the object usually appears in the partitive:

Päivitän profiilin. = I will update the (whole) profile.
En päivitä profiilia. = I will not update the profile.