This study is intended to help English foreign language learners avoid a type of syntactic ambiguity in sentences resulting from prepositional phrases (PPs). The students are guided to make use of the features of both adverbials and adjectivals in resolving this type of ambiguity. The movability of adverbials and the use of relative clauses instead of adjectival prepositional phrases are the two tricks that have helped students construct comprehensible sentences. The study avoids the use of syntactic tree diagrams since the relation among the words of the sentence should be clear before applying or drawing the syntactic tree diagrams. The students are given two assignments: the first after an introduction to the form and functions of PPs and the second after detailed instructions about the functions of PPs and how they can be a source of ambiguity in a sentence. This paper seeks to provide a practical solution to help learners avoid constructing ambiguous PPs. When people can infer more than one meaning, this ambiguity is considered a linguistic problem since language is used mainly to communicate thoughts and messages among people (Sturta, et al., 2003).