Please explain the role played by the word 'especially' in a sentence which begins with an independent clause and is followed by a comma and the word 'especially'. I have noticed that 'especially' often precedes a subordinating conjunction (especially if, especially when, especially since, etc.) In this case, is it just introducing the conjunction or clause? Is it, as an adverb, part of the subordinating conjunction?
What about other uses? Just to emphasize something?
The primary function of the adverb "especially" (also "chiefly", "mainly" etc.) is that of focusing modifier. It's not part of the subordinator, but a separate constituent that is frequently optional. It doesn't introduce the subordinate clause as such - that function is realised by the subordinator, which serves to mark the clause as subordinate.
It’s actually a special kind of focusing modifier called a ‘partial restrictive focusing modifier’ It’s called that because the restriction is only partial, meaning that the focus is not entirely on the clause it modifies, but the extent of the focus in relation to the preceding main clause.
They're not very mobile adverbs, mostly appearing immediately before the subordinator:
(1) I’m concerned about the climate in Cyprus, especially if I go in July.
(2) His performance was impressive, especially when you consider his age.
In ex (1), the focus is on the climate in July. It doesn't say that I wasn’t concerned with the climate in any month other than July, but rather that I wasn’t concerned about the climate in any other month to the same extent: any other concerns were relatively minor. In (2), the focus is on the age of the performer. It doesn't say that his performance wasn't impressive, but that it wasn’t impressive to the same extent that it was in view of his age at the time.
Focusing modifiers are not restricted to being modifiers in clause structure: they attach to a wide range of phrases too. This time, the preceding element may be a clause or a phrase, or even nothing at all:
(3) I was especially concerned about the cost.
(4) Kim loves taking long walks, especially in the park.
(5) I love most food, but especially fish.
In (3), "especially" modifies the AdjP "concerned about the cost". In (4), it modifies the PP "in the park" and in (5) it modifies the noun "fish".
Like (1) and (2), the partial nature of the restriction can easily be identified. For example, (3) does not say (as it would with fully restrictive "only") that I wasn’t concerned with anything except the cost, but rather that I wasn’t concerned with anything else to the same extent: any other concerns were relatively minor.