The Classification of Words

§ 387. In the sentence It rains the notional value of the subject is zero since it does not name or indicate any person, thing or idea. This is why it is (not quite adequately) called an 'impersonal' subject. But its predicative (structural) mean­ing is as good as that of any other subject: it shows that neither the speaker nor his interlocutors are involved.
In the sentence He is a student the notional value of is is next to zero, which prevents it from being recognized as the predicate of the sentence. Though is contains the tense and mood components of predicativity like any other predi­cate, it is regarded as only part of the predicate.
One cannot fail to notice that different criteria are used with regard to the subject and to the predicate. It is assumed that the former can be devoid of notional value, while the latter cannot.
When arguing against the traditional view that is in the sentence He is in Moscow is the predicate, A. I. Smirnitsky writes: "We cannot say that is is the predicate because the lexical meaning of this verb is colourless and indefinite" *.
The reason why modal verbs and other semi-notional verbs are not regarded as predicates is of the same nature.
§ 388. We think it essential to apply the same principles to the subject and predicate alike. The correlation between the structural and the notional in the principal parts of the sentence may be of four types: 1) The structural and the no­tional are united in one word.
E. g. Birds fly.
2) The structural and the notional are in different units.
I !
E. g. It is necessary to act.
l l
¹ See А. И. Смирницкий. Синтаксис английского языка. 1958, p. 113—115.

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3. 
   Only the structural is given in the sentence.
   E. g. Is it raining? It is.
   
4. 
   Only the notional is present.
   

E. g. What is he doing? Writing.
The differentiation of the structural and the notional is not an artificial device. As shown below, it is a characteris­tic feature of the analytical structure of the English sentence.