Learn Finnish: Opettaja mainitsi virheen tiedostossa. - The teacher mentioned the error in the file.

Mainita is a Type 5 verb (verbs ending in -ita or -itä). To make the past tense, you drop the final -ta and add the past marker -itsi-. Because this is the third-person singular form (he/she), we do not add any extra personal ending. So, mainita becomes mainitsi.

Why does the word for error, virhe, have an -n at the end here (virheen)?

It takes the -n ending because it is the total object of the sentence. The teacher mentioned the entire specific error, not just an unspecified amount of it. When an action is completed in a normal affirmative sentence, a singular object gets this -n ending.

If the base word is virhe and we add -n, why is there a double 'e' in virheen?

Finnish words that end in a single -e in their basic dictionary form (like virhe, huone, or osoite) follow a special rule. When you add an ending to them, you must double that final vowel first. So the stem of virhe becomes virhee-, and then you add the -n to get virheen.

What is the base form of tiedostossa and what does the ending mean?