Learn Finnish: Tämä paita on liian tahrainen kelvatakseen. - This shirt is too stained to be acceptable.

It is built from the 1st infinitive of the verb kelvata (to be acceptable), the translative marker -kse-, and the third-person possessive suffix -en. In this excessive resultative construction (too X to Y), the possessive suffix matches the subject of the sentence, which here is the third-person Tämä paita.

Could I use kelvataksensa instead of kelvatakseen?

Yes, you can. The third-person possessive suffix has two variants: the -nsA ending and the vowel-lengthening -Vn ending. Both kelvatakseen and kelvataksensa are grammatically correct and mean the exact same thing, but kelvatakseen is much more common in practice.

What exactly does the verb kelvata mean here?

Kelvata translates to 'to be good enough', 'to suffice', or 'to be acceptable'. It is an intransitive verb often used when evaluating if something meets a certain standard. Here, it implies the shirt is too stained to be worn or used for whatever occasion was intended.

How does the word tahrainen relate to other vocabulary?