In any question that expects a yes or no answer, the focus word is marked with the kO suffix. The focus word is the one that the question is about. When the focus word is the predicate, the question is whether the statement is true or not. Otherwise, the statement is otherwise expected to be true, but the question is whether it is true even when the focus word is present.
For example, a question that starts with “Do you know…” in English starts with Tiedätkö,… in Finnish. Here tiedät means “you know”, and it is turned to the focus word of a question by adding the kö suffix. Thus, when the kO suffix is appended to the predicate of a statement, it turns it to a question “Is it true that…?”.
When the kO suffix is appended to a word other than the predicate, the statement turns to question of the type “Is/was it ... that ...?”. Let us take the simple statement Liisa osti auton and form different questions by appending the suffix to different words: Liisako osti auton? (Was it Liisa who bought a car?), Ostiko Liisa auton? (Did Liisa buy a car?), and Autonko Liisa osti? (Was it a car that Liisa bought?). For example, Autonko Liisa osti? more or less implies that Liisa bought something, and it just asks whether it was a car.
Normally the focus word appears first in a clause, or at least in the first member of a clause. Although it is really the suffix and not the order that matters, here other orders would mostly sound odd, or maybe sometimes poetic. However, small conjunction-like words like siis (so, well) may appear before the member that has the interrogative suffix.
The idiomatic way to answer a yes/no question affirmatively in Finnish is to use the focus word, with the kO suffix removed. For example, the affirmative answer to the question like Ostiko Liisa auton? is Osti. This is described in detail in Asking and answering questions.
The kO suffix is often followed by the hAn suffix, as in Ostikohan Liisa auton? This is common in speech, to avoid asking a question too abruptly. The hAn suffix in this context somewhat corresponds to the English phrase “I wonder if…”.
Alternatively, the kO suffix may be followed by the s suffix. This is common when asking in 2nd person singular, e.g. Ostitkos sinä auton? (Did you buy a car), but it may be used otherwise, too. Often such a question more or less expects the answer to be affirmative and thus asks confirmation for something that has been heard.
In colloquial language, ks is often used instead of ko or kö, e.g. ostiks instead of ostiko. Originally, ks is a shortened form of kos or kös, which combines two word-like prefixes, and forms like ostikos are also still in use.
When the ks suffix is appended to a 2nd person singular verb form, the personal suffix t is omitted. This means that e.g. ostiks corresponds to both ostitko and ostiko.
On the other hand, such forms often combine with the personal pronoun sä (= sinä) after it, so instead of standard Finnish ostitko (or ostitko sinä), one says ostiksä.
A subordinate question (also called indirect question) is a question that is not actually being asked; instead, the main clause says something about the question. For example, “I don’t know what to do” contains the subordinate question “what to do”. In Finnish, we would say En tiedä, mitä tehdä. The subordinate question is the same as the corresponding real (direct) question, just written as a subordinate clause and without a question mark. It is always separated with a comma from the rest of the sentence.
However, for yes/no questions, the situation is different. If a real question like “Will it rain tomorrow?” is turned to a subordinate question, English changes the structure: “I don’t know whether it will rain tomorrow” or “I don’t know if it will rain tomorrow”. Finnish does not change the structure. The real question would be Sataako huomenna? and the subordinate question is identical: En tiedä, sataako huomenna.
When the interrogative suffix kO appears in a subordinate question, it cannot have the hAn or s suffix appended to it. It is normal to ask Sataakohan huomenna?, but it would be ungrammatical to say En tiedä, sataakohan huomenna.
Although Finnish mostly keeps yes/no questions as structurally different from other questions (see section Asking and answering questions), they can be mixed in spoken language, especially in some areas. A question like Oletko mihin aikaan tulossa? starts like a yes/no question, but it also contains the interrogative pronoun mihin (illative of mikä “what”), and it means the same as standard Finnish Mihin aikaan olet tulossa? (At what time are you coming?).