The suffix kä can be classified as a conjunction. It may be attached to forms of the negation verb (en, et, ei etc.), and it connects expressions the same way as the word ja “and”. For example, the sentence Ei sada eikä tuule is like Ei sada, ja ei tuule “It is not raining, and there is no wind”, except that the latter is not normal Finnish.
In standard Finnish, the negation verb is in a form required by the subject, e.g. Te ette syöneet ettekä juoneet (You did not eat or drink). However, it is not uncommon to use eikä as uninflected, e.g. He eivät syöneet eikä juoneet.
It is more or less automatic to use the suffix kä rather than the word ja when ja would start a clause that contains the negation verb. This applies to all forms of the negation verb, including imperative forms. Thus, normally enkä is used instead of ja en, and äläkä is used instead of ja älä.
The use of the suffix kä instead of the word ja is normal even when there would be a subject between ja and the negation verb. This causes a change of the word order. For example, instead of ja me emme tiedä we say emmekä me tiedä, putting the subject after the negation verb.
The suffix kä can also be used to connect nouns in a negative context. The sentence En halua kahvia enkä teetä means “I don’t want coffee or tea” or, expressed more clearly, “I don’t want coffee and I don’t want tea”. The sentence can be regarded as a shortened form of En halua kahvia enkä halua teetä.
The -kä conjunction can be used after a positive sentence to clarify it by refuting an alternative that might otherwise seem possible. For example, the statement Asun Espoossa enkä Helsingissä (I live in Espoo and not in Helsinki) is meaningful in a context where someone has shown to believe or suspect that I live in Helsinki. Using ja en instead of enkä would hardly be possible here.
The suffix -kä can be used at the start of a sentence when strongly opposing something that has been said or suggested. For example, when told to write something, one could say Enkä kirjoita! or Enkä kyllä kirjoita!
An expression containing ja can appear in a negative context, when it expresses close connection. For example, Se ei haittaa opetusta ja tutkimusta is possible, when opetus ja tutkimus (education and research) is treated as one concept. Sometimes ja connects synonyms, and one could say Se ei tunnu ja kuulosta hyvältä, since tuntuu ja kuulostaa (literally “feels and sounds”) is a phrase used to mean “seems”. However, such usage might be regarded as incorrect, and Se ei tunnu eikä kuulosta hyvältä might thus be safer.
The word pair ei … eikä (and en … enkä etc.), meaning “neither … nor”, is often presented as a combined conjunction. However, only the kä suffix is a conjunction here.
In expressions like Ei Paavo eikä Pekka tiedä sitä (Neither Paavo nor Pekka knows it), standard Finnish rules require that that the negation verb appears twice. However, it is not uncommon to omit the first negation verb at the start of a sentence: Paavo eikä Pekka tiedä sitä. This can be confusing, since there is the expression type Paavo, eikä Pekka, tietää sen (Paavo, not Pekka, knows it). Such a tendency is especially strong in sentences like Kukaan eikä mikään toimi tässä maailmassa yksin (Nobody and nothing works alone in this world), since the pronouns kukaan and mikään are always used in negative contexts and could be seen as carrying a negation; yet, standard Finnish says Ei kukaan eikä mikään toimi…
Some grammars describe this suffix as kA, but all forms of the negation verb require a front vowel, i.e. the suffix is kä. However, some pronouns can be interpreted as having a ka suffix, which may have a softening effect, e.g. milloinka instead of milloin (when). Such a suffix has little in common with the connective suffix kä, and it is best analyzed as just variation in the forms of some pronouns.