A subclause is, by definition, a constituent of another clause. For example, in the English sentence “He said that he loves you”, the subclause “that he loves you” is the object of the main clause “He said …”. The situation is the same in the Finnish sentence Hän sanoi, että hän rakastaa sinua. Note that in Finnish, commas are used more often between clauses than in English.
There are differences between languages when English uses a preposition before a subclause. There is no problem in writing “I inferred it from what I saw on TV”, where the subclause “what I saw in TV” together with the preposition ”from” constitutes an adverbial in the main clause “I inferred it …”.
In Finnish, you cannot use a preposition before a subclause. The equivalent of “to infer”, päätellä, requires that we use the elative (-stA) case, which here, as so often, corresponds to “from”. For example, we say Päättelin sen tästä dokumentista (I inferred it from this document). The problem with a subclause is that we cannot put an entire clause in a case form. The solution is to use the pronoun se as a grammatical device, as a word that gets inflected instead of a subclause that it relates to.
In order to inflect a mikä subclause, we use se, mikä instead of just mikä. By using different inflected forms of se, like the irregular elative siitä, we can effectively use a mikä subclause in any case form needed: Päättelin sen siitä, mitä näin tv:stä.
This way, the word se takes care of being in the grammatical form required by the main clause. The word mikä is put into the form required by the structure of the subclause itself, here in the partitive mitä, as needed for the object.
The pronoun se is similarly used with other types of subclauses, e.g. in se, joka and se, että. For example, to express trust in something, we can use the verb luottaa in Finnish, and it requires the illative case. To express “I trust that I get my salary tomorrow”, we combine the clauses Luotan and että saan palkkani huomenna using the illative form of se, i.e. siihen: Luotan siihen, että saan palkkani huomenna. An example of inflecting an että clause: Tämä johtui siitä, että talvi oli niin kylmä (This was caused by the fact that the winter was so cold).
In literary style, we could often use a clause equivalent instead of a subclause, e.g. Päättelin sen tv:stä näkemästäni. Here the subclause mitä näin tv:stä has been replaced by the participial construction tv:stä näkemäni, which can be inflected. However, this often leads to sentence structures that are more difficult to understand.
When the sentence context requires a subclause to appear in the nominative, the use of se is required when the subclause appears at the start of the sentence.
Se, että tähän rakennettiin tie, vaikutti paljon ratkaisuumme. (The fact that a road was built here affected a lot on our decision.)
In modern language, se is sometimes omitted (Että tähän vaikutti...), but this is clearly substandard.
If the subclause appears later, the use of se is optional and mostly a matter of style. Its use may slightly emphasize the subclause, or it may make the flow of words smoother.
Ratkaisuumme vaikutti paljon se, että tähän rakennettiin tie.
Ratkaisuumme vaikutti paljon, että tähän rakennettiin tie.
Similar considerations apply to subclauses in contexts where the partitive is required. This means that a subclause appearing after a main clause may act in the role of a partitive, even when no partitive suffix is present. Here the use of se, in the partitive form sitä, is not common, but possible.
En muistanut, että tänään on vaimoni syntymäpäivä. (I did not remember that today is my wife’s birthday.)
En muistanut sitä, että tänään on vaimoni syntymäpäivä.
When a subclause appears in the role of an object and the case required is the genitive, the use of se, in the genitive form sen, is optional. It is mostly omitted.
Muistatko, että tänään on vaimosi syntymäpäivä? (Do you remember that today is your wife’s birthday?)
Muistatko sen, että tänään on vaimosi syntymäpäivä?