The negative in Finnish is actually another verb form, although this one doesn't change for tense or mood. Each grammatical person, singular and plural, has its own form.
 

Thus, when answering "no" to a question, you must use the proper form of the negative verb:

• Puhutko suomea? Do you speak Finnish?
• En (puhu).    No(, I don't).

 
• Ostaako hän leipää?  Is he buying bread?  Is he going to buy bread?
• Ei (osta).            No(, he isn't).