Hi, everyone.
I'm wondering if I've understood the concept of 'grammatical subject' in the it-construction correctly and would be grateful for help. For example, in a sentence like, "It lay a dog on the bed" which clearly isn't right - would you call this an attempt to make "it" the grammatical subject? And on a similar note, if we change it into the more correct "There was a dog lying on the bed", does that make "there" the grammatical subject (and "a dog lying on the bed" the object)? Or am I barking up the wrong tree here, and am mixing grammatical subject up with something else?
Also, from what I've read, I gather that the reason for it being wrong to use the it-construction in cases like this is that the actual subject is a noun phrase instead of a clause. Am I right in this?

Thank you in advance.
"It lay a dog on the bed" does attempt, unsuccessfully, to make 'it' the subject.
The prepatory 'there' is never the subject of a sentence. "There was a dog lying on the bed." In this sentence the dog is the subject, because this is what the verb is happening to. The bed is the direct object.
Thank you for replying. "The bed" is the direct object? Really? I would've thought it an adverbial, since it answers the question 'Where?' Any chance you could explain what it is that makes it the direct object?
So "there" can't be a subject. Got it. Is it something else? I mean, does it actually have a grammatical function as a clause element in the sentence? 
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The bed is not the direct object. A direct object will never be in a prepositional phrase. Apologies for misleading.
You are right. The introductory 'there' doesn't fit grammatically with the rest of the sentence.