Minä kuumennan kahvia.

Because -n is the 1st person singular ending in Finnish. It shows that the subject is I.

• kuumentaa = to heat
• kuumennan = I heat / I am heating

So even without minä, the verb already tells you the subject is I.

Not always. Finnish often leaves the subject pronoun out when the verb ending already makes the subject clear.

So both of these are possible:

• Minä kuumennan kahvia.
• Kuumennan kahvia.

The version with minä can sound more explicit or emphatic, like I am heating coffee.

The dictionary form is kuumentaa, meaning to heat.

This sentence uses the present-tense 1st person singular form:

• kuumentaa → kuumennan

So if you look the word up in a dictionary, you would search for kuumentaa.

Because kahvia is the partitive singular form of kahvi.

In Finnish, the object is often in the partitive when:

• you mean an unspecified amount
• the action is ongoing, incomplete, or viewed as a process
• the noun is a substance like coffee, water, milk, etc.

So:

• kahvi = coffee
• kahvia = some coffee / coffee (as a substance)

In Minä kuumennan kahvia, the idea is something like I am heating some coffee or I am heating coffee.

Kahvin is the accusative/genitive-looking total object form here, and it would usually suggest that the coffee is seen as a whole, completed object.

Compare:

• Minä kuumennan kahvia. = I am heating coffee / some coffee
• Minä kuumennan kahvin. = I heat up the coffee / I will heat the whole coffee portion

This is a very important Finnish difference:
partitive object often means ongoing, partial, or indefinite, while the total object often means completed or definite.

Yes. Kuumennan is in the present tense.

Depending on context, the Finnish present tense can mean:

• something happening right now: I am heating coffee
• something habitual: I heat coffee
• sometimes even near future: I’ll heat the coffee

Without context, Minä kuumennan kahvia is most naturally understood as I am heating coffee or I heat coffee.

Yes. Finnish word order is fairly flexible, although not completely free.

The most neutral order is:

• Minä kuumennan kahvia.

But you can also say:

• Kahvia kuumennan.
• Kuumennan kahvia.

Changing the word order usually changes emphasis, not the basic meaning.

For example:

• Kuumennan kahvia. = neutral, common
• Kahvia kuumennan. = emphasizes coffee
• Minä kuumennan kahvia. = emphasizes I, or just sounds a bit more explicit

A useful way to think of it is:

• dictionary form: kuumentaa
• stem: kuumenna-
• 1st person ending: -n

So:

• kuumenna- + n = kuumennan

This is the normal pattern for many Finnish verbs: take the verb stem and add the personal ending.

A simple pronunciation guide is:

• Minä ≈ MIH-nah
• kuumennan ≈ KOO-men-nahn
• kahvia ≈ KAH-hvee-ah

A few helpful points:

• Finnish stress is usually on the first syllable
• uu is a long vowel, so kuu- is held a bit longer
• nn is a long consonant, so you hold the n slightly longer
• Finnish pronunciation is usually very regular compared with English

Yes, but in casual speech the pronoun may be dropped:

• Kuumennan kahvia.

In very colloquial spoken Finnish, many speakers might also say:

• Mä kuumennan kahvia.

Here mä is the everyday spoken form of minä.

So:

• Minä kuumennan kahvia. = standard, clear Finnish
• Kuumennan kahvia. = also very natural
• Mä kuumennan kahvia. = colloquial spoken Finnish