There are more than 300 different definitions of the sentence, but not a universal one

30. The principal sentence parts

The principal parts of the sentence are the subject and the predicate, which modify each other: the subject is the “person” modifier of the predicate, and the predicate is the “process” modifier of the subject; they are interdependent. 

Subject. The subj.is a word or a group of words denoting the thing we speak about.. s. & p. are of the same syntactic rank
BUT! W. Chafe believes that there is only one principle part- the predicate! As the subj. is one of the verb compliments! It may be regarded even to the same level as obj. The verb determins the form of both subj.and obj. ( sees >he>>me)
Arguments in favour!
1.In the majority of cases the verb is present.
2. the verb determines what the rest of the sentence will be like. ( the chair laughs. Глагол делает подлежащие одушевленным)
Arguments that subject is one of the principle parts.(ильицкий? Не уверена, не смогла у Лены разглядеть)

1. 
   The s. is the only sent.part, which not only subordinate to the predicate, but also subordinate the pred. (he writes)
   
2. 
   in the pred-obj. relation the form of the obj is determined lexically. (I saw him) the obj. case depends on the lexical meaning of the verb, whereas the nom. Case of the subj. doesn’t depend on the lexical meaning of the predicate. The s-pr relations are syntactically conditioned. While pred-obj relations are lexically conditioned.
   
3. 
   The sub. Position is morphologically marked, the s. is always in the nom. Case and preceeds the pr. The obj position is morphologically unmarked. The obj. may be in the obj.case preceeded by preposition
   
4. 
   The sub.position is always obligatory
   

Problems

1. 
   The problem of complex members( comp.subj, complex obj)
   

Pr. Ильиш is against the complex subject ( he is seen to enter the room) the splitting of the subj. is strange to English. In his opinion he is the subj. & was seen to enter- a peculiar type of the compoungd predicate

2. 
   The linguistic status of IT and THERE in personal & existential sentences is problematic! Some scholars say that it,there are part of the subj. роговская, хаймович name them them “secondary subj” in existential sent-s (introductory)
   

Predicate
Problems

1. 
   The problem of a simple nominal predicate.
   

Trad-ly “simple nominal pred”is recognized,
but ИЛЬИШ > 2 varieties of it:

1. 
   Pred follows the subj. ( my ideas obsolete)
   
2. 
   Pred preceeds the subj. (splendid game cricket )
   

Смирницкий speaks about the following types of pred. from the point of view

1. 
   Of the structure ( simple, compound)
   
2. 
   Of meaning ( qualificative, objective-I have a lot of friends, adverbial-he is here, process –I can see nothing, double( a blend of process & qualification термин смирницкого) –the snow fell thick)
   

Curme “ a grammar of English lg”-
1)the predi is expressed by a finite form of complete predication
2) the pred-is ex-d by a verb of incomplete predication + complement! (object or predicative) – man is mortal –
3)finite form+opposition he came home sick