I found the following sentence while I was going through a tutorial. Though I can understand the meaning of the sentence clearly, I cannot analyze the sentence grammatically. The reason for this is that there is no explicit subject for the first part of the sentence. Therefore, I would appreciate it if someone could explain the grammar associated with the following sentence.
When offered chips, I always choose the cheesiest ones.
[1]
When offered chips, I always choose the cheesiest ones.
Grammatically, modern grammar classifies "when" as a preposition. On that analysis, the element in bold is a preposition phrase where the head ,"when", has the passive non-finite clause "offered chips" as its complement. It has a paraphrase containing noun+defining relative clause:
On occasions when offered chips, I always chose the cheesiest ones.
[2]
When I am offered chips, I always choose the cheesiest ones.
It's common for certain grammaticised words at the beginning of a clause to be omitted, especially in casual style. Most non-finite clauses do not have an overt subject, but in this case there is also ellipsis of an auxiliary verb. The missing subject is the pronoun "I", while the missing auxiliary verb is "be". Adding the subject and filling out the ellipsis gives a finite clause as complement of "when", as in [2],
