Why I cannot use universal instantiation rule here? I have the following premise: 

$$\forall x (Fx \vee \neg Fx) \rightarrow Ga$$

Why I cannot use UI and get: $(Fa \vee \neg Fa) \rightarrow Ga$
And what is the difference between the first premise and the below premise:

$$\forall x ((Fx \vee \neg Fx) \rightarrow Ga)$$

Thanks in advance.
 A: You cannot use UI since you can only use UI on statements that are universals. But that first statement is not a universal, but a conditional, whose antecedent is a universal. Maybe it helps you to see that if I add another set of parentheses:
$(\forall x (Fx \lor \neg Fx)) \rightarrow Ga$
Indeed, the difference with the second statement is that the second statement is a universal, and so you can apply UI to that statement to get $(Fa \lor \neg Fa) \rightarrow Ga$
In general, you want rules of inference to apply only to statements as a whole. For example, take Simplification:
$P \land Q$
$\therefore P$
If you try to apply Simplification to the component statement $A \land B$ of the larger statement $(A \land B) \rightarrow C$, the following happens:
$(A \land B) \rightarrow C$
$\therefore A \rightarrow C$
But that is clearly not valid! That's why inference rules can only be applied to statements as a whole, and not to component statements.
A: 
And what is the difference between the first premise and the below premise

Suppose you are trying to get a date to a dance.
"If every girl is spoken for, then I am going alone."
"For any girl, if she is spoken for then I am going alone."
Note that in the first case, as long as any girl isn't asked you might have a chance.  In the second case, if any girl has made a commitment then you can't get a partner at all.  The second statement is equivalent to:
"If any girl is spoken for, then I am going alone."
Quite different from the first statement.
