1. Structural ambiguity 

Definition: Structural ambiguity is one or more string(s) of words has/have more than one meaning. For example, the sentence Tom said he would come yesterday can be interpreted in different ways.

2. Word order 

Definition: Different arrangements of the same words have different meanings. For example, with the words Tom, love and Mary, we may say Tom loves Mary or Mary loves Tom. •

3. Grammatical relations 

Definition: Native speakers know what element relates to what other element directly or indirectly. For example, in The boats are not big enough and We don’t have enough boats, the word enough is related to different words in the two sentences.

4. Recursion 

Definition: The same rule can be used repeatedly to create infinite sentences.

For example, I know that you are happy.

He knows that I know that you are happy.

She knows that he knows that I know that you are happy. •

5. Sentence relatedness 

Definition: Sentences may be structurally variant but semantically related. •

6. Syntactic categories 

Definition: A syntactic category is a class of words or phrases that can substitute for one another without loss of grammaticality.

For example, consider the following sentences: 

The child found the knife. 

A policeman found the knife. 

The man who just left here found the knife. 

He found the knife.

All the italicized parts belong to the same syntactic category called noun phrase (NP). The noun phrases in these sentences function as subject. The knife, also a noun phrase, functions as object.