-ko/-kö

To form a Yes-No question in Finnish, follow these steps:

1. Choose the focus: Usually, this is the verb. You must use the conjugated form, not the dictionary form. For example, to ask 'Do you speak?', start with puhut (you speak).
2. Move it to the front: The host normally belongs to the initial questioning constituent of its clause. While the verb is most common, you can move any word to the front to focus the question on that specific thing (e.g., Sinäkö puhut? = 'Is it you who is speaking?'). Note that conjunctions like mutta (but), ja (and), tai (or), or the subordinating conjunction että (that) can precede it.
3. Apply Vowel Harmony:

• Use -ko if the word has back vowels (a, o, u). Example: puhut → puhutko?
• Use -kö if the word has front vowels (ä, ö, y) or only neutral vowels (e, i). Example: syöt → syötkö?
• For compound words, vowel harmony follows the last word in the compound (e.g., työmaa → työmaako?).

4. Negative Questions: To ask 'Isn't...?' or 'Don't...?', attach the suffix to the negative verb (e.g., ei + kö = eikö).

5. Placement: The suffix is usually the last part of the word, coming after personal endings, case endings, and possessive suffixes (e.g., auto + ssa + ni + ko = autossaniko? 'in my car?'). In more complex speech, if multiple clitics are used, -ko/-kö can combine with other clitics. Both orders with additive -kin are possible, with different emphatic readings: tätäkinkö? and hänkökin?. Common following particles include -han/-hän and -s, as in onkohan? and onkos?; do not freely combine all particles into a single fixed-order recipe.

6. Consonant Gradation: This suffix does not trigger consonant gradation. The word stays in whatever grade it was already in (e.g., pankki becomes pankkiko?).

7. Pronunciation: The -ko/-kö suffix is unstressed and does not shift the primary stress from the first syllable of the word (e.g., PU-hut-ko).

8. Spoken Finnish: In spoken Finnish (puhekieli), -ko/-kö is frequently shortened to -ks (e.g., Oletko? → Ootko? → Ootks? / Ooksä?).

9. Warning: Do not use -ko/-kö with question words (WH-words like kuka, mikä, missä) to form a standard question (e.g., 'Mikä tämä on?' NOT 'Mikäkö tämä on?').

Answering: While you can say joo or ei, it is more natural to answer with the verb. To answer positively, repeat the affirmative verb conjugated to the correct person (e.g., Puhutko? → Puhun.). To answer negatively, it is very common to answer with just the conjugated negative verb (e.g., En.). You can also use the conjugated negative verb (en, et, ei) followed by the main verb stem for present tense answers (e.g., Puhutko? → En puhu.), or the past participle for past tense answers.

Negative questions agree with the subject: enkö, etkö, eikö, emmekö, ettekö, eivätkö. For example, Etkö puhu suomea? and Eivätkö he tule? The main verb is the connegative form in the present. As a conversational tag, eikö niin? is a separate expression.
Echoing a question: Ordinary information questions use Mikä tämä on? Adding -kö gives a different use: Mikäkö tämä on? echoes or takes up what someone has asked, rather than being ungrammatical in every context.