How to determine whether an ambiguous sentence or phrase is syntactically ambiguous:

1. Determine which readings the sentence or phrase has (paraphrasing helps).
   
2. Check the syntactic category of each word in each of the readings. If one or more words has a different syntactic category in different readings, the sentence or phrase exhibits lexical syntactic ambiguity.
   
3. Else, check the syntactic structure of the sentence or phrase in each of the readings (that is, draw tree diagrams; you may draw a partial tree involving only the ambiguous parts, if you feel confident enough). If the diagrams are different, the sentence or phrase exhibits structural syntactic ambiguity.
4. Else, the sentence is not syntactically ambiguous (it may still be semantically, pragmatically, or otherwise ambiguous).
   

Note that what I just wrote resembles a recipe or a set of instructions for assembling a piece of furniture. By breaking down a complex process into a sequence of smaller, well defined and ordered steps, we are carrying it out through what is called an algorithm.

Practice exercise: Make sure you understand why the following sentences exhibit the type of ambiguity indicated (follow the algorithm if you don't).

• He saw her duck: lexical syntactic ambiguity (also structural, of course).
• I saw the man with the telescope: structural syntactic ambiguity.
• I drove into the bank: no syntactic ambiguity (in this case the ambiguity stems from the lexical semantics, or word meaning, of bank).