Tämän kahvinkeitimen takuu on vielä voimassa.

Because they form a genitive noun phrase meaning of this coffee maker:

• tämä (this) → genitive tämän
• kahvinkeitin (coffee maker) → genitive kahvinkeitimen

In Finnish, demonstratives and adjectives agree in case and number with the noun they modify, so tämän matches kahvinkeitimen. The whole genitive phrase tämän kahvinkeitimen then modifies takuu: the warranty of this coffee maker.

Finnish often stacks modifiers before the head noun. Here, the structure is:

• [tämän kahvinkeitimen] takuu = the warranty [of this coffee maker] Then the verb phrase:
• on vielä voimassa = is still valid

So the natural grouping is tämän kahvinkeitimen takuu as one noun phrase.

The dictionary form is kahvinkeitin. Its genitive singular is kahvinkeitimen.

This is a regular declension pattern for many -in nouns:

• nominative: kahvinkeitin
• genitive: kahvinkeitimen
• partitive: kahvinkeitintä

So -in → -imen in the genitive is just how this noun type inflects.

It is a compound noun, and Finnish compounds are usually written as one word. It’s built from:

• kahvin = genitive of kahvi (coffee)
• keitin = device for brewing/boiling (a brewer/maker)

So kahvinkeitin literally means coffee-brewer/maker, i.e. coffee maker. Using the genitive kahvin- as the first part is very common in Finnish compounds.

In this expression, voimassa functions like an adverbial/state phrase, not a typical adjective. Finnish often expresses states with case forms like this:

• on voimassa = is valid/in effect There are adjectives related to validity (like voimassa oleva = currently valid, literally being in force), but for warranties the idiomatic everyday choice is on voimassa.

vielä means still: the warranty has not expired yet.

It’s flexible, but these are common:

• Takuu on vielä voimassa.
• Takuu on voimassa vielä. (often with contrast/emphasis: it’s valid still, at least for now)

Close alternatives:

• yhä / edelleen = also still, slightly more formal/neutral than vielä in some contexts.

Common negatives are:

• Tämän kahvinkeitimen takuu ei ole enää voimassa. = The warranty is no longer valid.
• Tämän kahvinkeitimen takuu ei ole vielä voimassa. = The warranty is not valid yet. (different meaning)

Notice:

• on → ei ole
• vielä (still/yet) vs enää (anymore) changes the meaning.

Invert the verb to the front and add -ko/-kö:

• Onko tämän kahvinkeitimen takuu vielä voimassa? = Is the warranty for this coffee maker still valid?