The following sentence is ambiguous. Provide two explanations which illustrate the different meanings of the sentence: I saw her duck.

The problem with this sentence is that it is unclear if I saw an animal ("her duck"), or if I saw her perform the action of ducking. Let's break each possibility down grammatically.
The first possibility: "duck" could be the direct object of the sentence. A direct object is the person or thing that receives or is affected by an action. In this case, the action is "saw," simple past form of "see." So, if I say, "I saw her duck," then I am referring to my encounter with the pet duck of some unnamed girl or woman. The phrase "her duck" takes the form of a verb complement. The complement is necessary in order to know what "I saw."
The second possibility: "duck" is used as a verb to express "her" action. In this sentence, "I" is the subject, "saw" in the main verb, and "her" is the direct object. "Duck" is the action I saw her perform.
This sentence could be revised for clarity in two ways: "She ducked" ('duck' is an intransitive verb), or "I saw her ducking," which transforms 'duck' into a gerund, or the thing I saw her doing.