The writer/speaker wants to change the interlocutors’ focus on different objects in the sentence. According to Chomsky (1971), a focus is one predicate inside a sentence. Since a sentence can contain more than one focus, contrastive focus arises to select a referent (centre of focus) from a set of possible referents signified in a sentence (Cowles et al., 2007).
Examples in the question are utilized for elucidation.
Subject-predicate: I (subject) buy you the cake (predicate).
Semantic roles: I (agent) buy you (benefactive) the cake (theme).
Subject-predicate: I (subject) buy the cake for you (predicate).
Semantic roles: I (agent) buy the cake (theme) for you (benefactive).
As more attention is paid to new information, the new information added is usually the contrastive focus. The contrastive focuses in the example sentences are identified and explained with the predicate and semantic roles above.
Sentence 1: ‘the cake’
Reason: incomplete meaning without ‘the cake’.
Since the action of ‘buy’ requires an object following it, adding ‘the cake’ completes the meaning and enables interlocutors to know what connects the action. After that, ‘buy’ is a transitive verb. So, it is the main focus.
Sentence 2: ‘you’
Reason: ‘you’ added new information to buy, allowing the interlocutors to know who benefits. So, it is the focus.
To conclude, the existence of different forms but the same meaning sentences is to fulfil the writer/speaker’s goal of changing focus of interlocutors. So, adding new information can switch focus and complete the sentence’s meaning, creating contrastive focus.
To identify the true focus (contrastive focus), individuals can start with the sentence’s meaning, subject with predicate and semantic roles.