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

31. Secondary sentence parts.
he secondary parts are: the object – a substance modifier of the predicate; the attribute – a quality modifier of substantive parts, either the subject or the object; the adverbial modifier – a quality modifier of the predicate; the apposition – a substance modifier of the subject; the parenthesis (parenthetical enclosure) - a detached speaker-bound modifier either of one of the nominative parts of the sentence or of the sentence in general; the address (addressing enclosure) – a modifier of the destination of the whole sentence; the interjection (interjectional enclosure) – an emotional modifier.
Do not form predication, they are used to complete&expand principle sentence-parts> they are called “compliments”.

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  Ac. Пешковский doesn’t use the notions of obj.,attr.,adv.modifier, but he classifies them into
  
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  which are governed
  
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  which are not governed.
  

Object. Tr-ly it is a secondary sent part, denotes a thing by the process or property, ex-d mainly in the predicate. An object is a noun phrase or a clause with nominal function( subdivided into simple, complex (when the object is expressed by an infinitival participial or gerundial complex),direct (a noun that completes the meaning of a transitive verb), indirect(denotes a living being to whom the action of the verb is directed)
Бурлакова speaks about O1, Obj2, Obj 3:
object 1- always non-prepositionalirrespective of its position in the sentence ( they sent a doctor away. They sent away a doctor)
2- eiter prepositional or non-prep. Depends on its position ( they sent a doctor some money. They sent some money to the doctor)
3- Is always prepositional ( they sent for the doctor. For the doctor they sent)
Symmetrical verbs- whose subj-s and obj-s may exchange their positions without producing any change in the meaning of the sent(.ex. to meet, to marry) there are some verbs not requiring any ob. In the sent. As the ob. Is mentioned in the verb! –go blackberry( ходить за ягодами)
Problems of the complex obj. – some scholars do not recognize an obj!