It is noticeable that—
(a) If the subject is in the sing., and denotes a living being, the complement must be in the nominative; one must say ihminen on kuolevainen, not kuolevaista.
(b) If the subject denotes a part of the body, the complement must be also in the nominative: Hiukset ovat mustat, the hair is black. Kädet ovat tarpeelliset, hands are necessary.
III. The complement is put in the essive if it denotes the state in which a thing is at a given time. Thus, isäni on kipeänä, my father is (now) ill; isäni on kipeä, my father is an invalid. Hän on pappi, he is a clergyman; but hän on jo kauan ollut pappina, he has long been a clergyman. Suomenmaa on osa Venäjän valtakuntaa, Finland is a part of the Russian empire; but tähän vuosisataan asti Suomenmaa oli osana Ruotsin valtakunnasta, till the beginning of this century Finland formed part of the kingdom of Sweden. Often there is little difference between the nominative and essive. Thus one can say either sydän on suruja täysi, or täynnä, the heart is full of woe; but täynnä gives a more precise and literal idea of fullness than täysi.
IV. With the other auxiliaries, signifying to become, or pass into a state (tulla, ruveta, muuttua, etc.), the complement is put in the translative. Ilma muuttuu lämpimäksi, the weather grows warm. Poika rupesi palvelijaksi, the boy began to be a servant. Ukko käy heikoksi, the old man gets weak. Hän joutui työmieheksi, he became a workman.
Such sentences as it is impossible to go, or it is necessary for you to go, may be rendered in Finnish as mahdotonta on mennä, or tarpeellista on että menette; but in both cases the real subject is the infinitive, or the sentence with että which replaces it.
In such sentences the adjective, combined with the verb olla, can be put either in the partitive or in the nominative; but it is very hard, not only to give rules for the employment of the two cases, but even to state the exact difference of meaning between them. As has been seen above, kivi on kova means the stone is hard, but kivi on kovaa, the stone belongs to the class of hard things. The same distinction appears to prevail in the case under consideration, but, as the difference is very subtle, it is not surprising that it is often hard to trace. On the whole, the partitive is more usual, because the use of the nominative implies, strictly speaking, that the subject and the complement are co-extensive. But the nominative is more definite, inasmuch as it vaguely implies the existence of particular personal or temporal conditions. Thus, parasta on mennä is the equivalent of it is best to go; and it is perfectly logical to use the partitive, as clearly it is not meant that the terms going and best are co-extensive. But paras on mennä means rather the best thing for us under the circumstances is to go; and in this case the use of the nominative is also logical, because the two terms are co-extensive. It can be easily imagined that with so slight a distinction the nominative and partitive are used almost indifferently in most cases. Thus, it is equally correct to say on surkea nähdä sinua tuossa tilassa, or on surkeata, it is sad to see you in this position. But it is noticeable that—
(1) Parempi and hyvä are always used in the nominative. Parempi on odottaa, it is better to wait. Hyvä on olla terveenä, it is good to be healthy. In the case of parempi, at any rate, this is quite natural, as there can hardly be said to be a class of better things without reference to particular circumstances. But this point cannot be pressed, as other comparatives are used in the partitive.
(2) In such phrases as on mahdotonta, it is impossible; onko luvallistra, is it allowable; onko mahdollista, is it possible? the partitive is nearly always used.