4 bit ALU variable name question

Im writing code for a 4bit Arithmetic Logic Unit in Verilog HDL as a 
structural model. I need help programming the function for adding 
alu_in_a+alu_in_b+Cin. I also need help with variable names. In this 
project the ALU module is intended to have inputs alu_in_a and alu_in_b. 
I designed all of my adders using A and B as inputs. I realized I don't 
fully understand how module instantiation inside another module works, 
and how inputs to one module are passed to another module for some 
desired operation. I would very much appreciate clarification on this in 
general and in the specific instance of the 4bit ALU. On a side note 
this code does compile, but I suspect there are errors resulting from my 
ignorance which will cause it to malfunction in simulation. Thanks!


// ALU

module ALU (input [3:0] alu_in_a, alu_in_b, OPCODE, input Cin, output 
reg [3:0] Sum, output reg Cout, output OF);

  reg [3:0] Bin;
  wire [3:0] Bn, S;
  wire Co;
  com2s C1 (aluin_b, Bn);
  ripple_adder_4bit FA4 (A, Bin, Cin, S, Co, OF);
  always @ (*) begin
    Bin = 4'b000; Sum = 4'b0000; Cout = 'b0;
    case (OPCODE)

      //ALU operations

      // A+B : add
    4'b1000 : begin
      Bin = alu_in_b; Sum = S; Cout = Co;
      end

      // A+B+Cin : add with Cin
    4'b1001 : begin
      Bin = alu_in_b; Sum = S; Cout = Co; ***HELP***
      end

      // A-B : sub a from b
    4'b1010 : begin
      Bin = Bn; Sum = S; Cout = Co;
      end

      // ~(A&B) : bitwise NAND
    4'b0000 : begin
      Sum = ~alu_in_a&~alu_in_b;
      end

      // ~(A|B) : bitwise NOR
    4'b0001 : begin
      Sum = ~alu_in_a|~alu_in_b;
      end

      // A^B : bitwise XOR
    4'b0010 : begin
      Sum = alu_in_a^alu_in_b;
      end

      // ~A : bitwise inversion
    4'b0100 : begin
      Sum = ~alu_in_a;
      end

      // A_leftshift
    4'b0101 : begin
      Sum = alu_in_a << 1;
      end
    endcase
  end
endmodule


// 4 Bit Ripple Carry Adder

module ripple_adder_4bit(input Cin, input [3:0] A, B, output [3:0] Sum, 
output Cout, OF);

 full_adder FA1(Sum[0], Cout1, A[0], B[0], Cin);
 full_adder FA2(Sum[1], Cout2, A[1], B[1], Cout1);
 full_adder FA3(Sum[2], Cout3, A[2], B[2], Cout2);
 full_adder FA4(Sum[3], Cout, A[3], B[3], Cout3);
xor X1 (OF, Cout3, Cout);

endmodule

// Twos Compliment

module com2s (input [3:0] B, output [3:0] Bn);
wire [3:0] Bn1;
wire OF;
assign Bn1 = ~B;
ripple_adder_4bit FA4 (Bn1, 4'b0000, 1'b1, Bn, Cout, OF);

endmodule

// Full Adder

module full_adder(output S, Cout, input A, B, Cin);
 wire Sum1, Cout1, Cout2;
 half_adder HA1(Sum1, Cout1, A, B);
 half_adder HA2(Sum1, C, Sum, C);
 or O1(Cout, Cout1, Cout2);

endmodule

// Half Adder

module half_adder(output Sum, Cout, input A,B);
 assign Sum = A^B;
 assign Cout = A&B;

endmodule