Yes, even if their construction doesn’t seem to be simple, these two sentences that you presented are actually simple sentences:
(1) “The Sydney Harbor Bridge was built during the Great Depression of the 1930s.”
(2) “During the Great Depression of the 1930s, the Sydney Harbor Bridge was built.”
Sentence 2 is, of course, simply the inverted version of Sentence 1; it puts the modifying phrase “during the Great Depression of the 1930s” ahead of the clause “the Sydney Harbor Bridge was built.” Either way, however, the meaning of the statement remains the same.
But I’m sure that the question that nags you and a lot of other people is this: How could those two sentences be simple when, in fact, their common clause, “the Sydney Harbor Bridge was built,” has a phrase attached to it? Doesn’t that added grammatical element make the sentence other than simple?
Well, the answer is no. That attachment is only a prepositional phrase acting as a modifier; without an active verb of its own, “during the Great Depression of the 1930s” doesn’t qualify as a dependent or subordinate clause. You will recall that by definition, a simple sentence has only one independent clause and no dependent or subordinate clauses whatsoever. When one or more dependent clauses are attached to a main clause, the sentence ceases to be simple. It becomes a compound sentence, complex sentence, or compound-complex sentence depending on the grammatical and logical relationship between the main clause and those clauses.
In reply to your last question,
“Lesson #5 – Constructing the English Sentence” in the Forum identifies the basic grammatical structures, namely the various types of phrases and clauses, that are used to form sentences. For an intensive discussion of the various kinds of sentences and their construction, though, I suggest you check out my book
Give Your English the Winning Edge. It devotes seven sections to the construction of sentences in English, including how to combine and link ideas and establish their logical relationships.
