Learn Hardcore Finnish: Minä haluan kohdella ystävää hyvin.

Both mean the same thing. Including minä adds a bit of emphasis to I (for contrast, insistence, or clarity), but in neutral speech you’d normally just say Haluan kohdella ystävää hyvin.

Why is it haluan and not haluta in this sentence?

Haluta is the dictionary (infinitive) form: to want.
In the sentence, it must be conjugated for the subject I:

minä haluan – I want
sinä haluat – you (sg) want
hän haluaa – he/she wants
me haluamme – we want
te haluatte – you (pl) want
he haluavat – they want

So haluan is 1st person singular present tense, matching minä. Using haluta here (Minä haluta kohdella…) would be ungrammatical.

Why is kohdella in the basic form and not conjugated?

In Finnish, verbs like haluta (to want), voida (can), pystyä (to be able), aikoa (to intend) are often followed by another verb in the infinitive (dictionary) form.

Pattern:

Haluan
- kohdella = I want to treat
Haluan
- syödä = I want to eat
Haluan
- oppia = I want to learn

So only the first verb (haluan) is conjugated; the second verb (kohdella) stays in its basic infinitive form to express to treat.

What exactly does kohdella mean, and how is it used?

Kohdella means to treat someone (in some way), to behave towards someone. It always requires some kind of object (the person being treated) and usually an adverb or expression that says how they are treated.

Examples:

Kohtele häntä hyvin. – Treat him/her well.
He kohtelevat minua huonosti. – They treat me badly.
Työnantaja kohtelee työntekijöitä reilusti. – The employer treats the employees fairly.

In your sentence, ystävää is the person being treated, and hyvin describes the way of treating them.

Why is it ystävää and not ystävä?

Ystävä is in the nominative (dictionary form).
Ystävää is partitive singular.

Here it’s partitive for two reasons:

Object of the verb kohdella
The verb kohdella virtually always takes the partitive for its object when it refers to a person:
- Kohtele minua hyvin. – Treat me well.
- Haluan kohdella ystävää hyvin. – I want to treat a/the friend well.
Indefinite, non-countable type of action
Kohdella describes how you behave toward someone in general, not a single, completed event. This kind of atelic/stative action in Finnish very often uses the partitive object.

So ystävää is required here; ystävä (nominative) would simply be wrong in this structure.

How do I say “my friend” instead of just “a friend” here?

You need a possessive form of ystävä in the partitive case, because kohdella still governs the partitive.

Correct options:

Haluan kohdella ystävääni hyvin. – I want to treat my friend well.
Minä haluan kohdella ystävääni hyvin. – same, with an explicit minä.

Breakdown of ystävääni:

stem: ystävä-
partitive ending: -ä → ystävää
possessive suffix -ni (my): ystävääni

You could also say:

Haluan kohdella minun ystävääni hyvin.

Here minun is a separate possessive pronoun, and ystävääni still has the -ni suffix.
Using minun ystävää without -ni on the noun is not standard in careful written Finnish.

Why do we use hyvin and not hyvä or hyvää?

Hyvin is the adverb form of hyvä (good) and means well.

hyvä = good (adjective, modifies nouns)
- hyvä ystävä – a good friend
hyvin = well (adverb, modifies verbs/adjectives)
- kohdella hyvin – to treat well
- Hän voi hyvin. – He/She is doing well.

Hyvää is the partitive of hyvä and is used in different structures:

Hyvää päivää. – Good day.
Haluan jotain hyvää. – I want something good.

Since in your sentence hyvin modifies the verb kohdella (how you treat someone), you must use the adverb: hyvin, not hyvä or hyvää.

Can I change the word order in this sentence?

Yes, Finnish word order is quite flexible, though the neutral version is:

(Minä) haluan kohdella ystävää hyvin.

Other possible orders, with different emphasis:

(Minä) haluan ystävää kohdella hyvin.
Puts slight emphasis on ystävää.
(Minä) haluan kohdella ystävää hyvin.
– neutral, most typical.
Ystävää haluan kohdella hyvin.
Emphasises ystävää (“It’s the friend that I want to treat well”).

You normally keep haluan kohdella together as a verb phrase, and hyvin stays near the verb it modifies. The version you will hear most in everyday speech is:

Haluan kohdella ystävää hyvin.

Is there something special about the case used with kohdella?

Yes. Kohdella is one of those verbs that normally require the object in the partitive, especially when the object is a person.

Typical patterns:

Kohtele häntä hyvin. – Treat him/her well.
He kohtelevat minua epäoikeudenmukaisesti. – They treat me unfairly.
Opettaja kohtelee oppilaita tasapuolisesti. – The teacher treats the students equally.

In all of these, the object (person) is in the partitive: häntä, minua, oppilaita, ystävää.

For this verb, a genitive or nominative object is generally not used in normal “treat someone (in some way)” meanings, because the action is not viewed as a bounded, completed event; it’s more like an ongoing manner of behaviour. So ystävää is the expected, almost obligatory form.

How would I say “I want to treat my friends well” (plural)?

You pluralise ystävä and still keep the partitive object, because kohdella wants the partitive.

Haluan kohdella ystäviäni hyvin. – I want to treat my friends well.

Breakdown:

ystävä – friend (nominative singular)
ystäviä – friends (partitive plural)
ystäviäni – my friends (partitive plural + possessive suffix -ni)

Without “my”:

Haluan kohdella ystäviä hyvin. – I want to treat friends well. (friends in general)