instance inside always block in verilog

Question

i have 4 module in active hdl

module1(ZAKAH): to calculate 2.5% from 10 input switch binary numbers and display on 7 segment display useing 3 output F0,F1,F2.

moudule2(PT): 1 input push button every clk will display constant time and give F0,F1,F2,F3 as a output to display on 7-Segment display

module3(BCD_counter): 1 input push button to counter from 000 -> 999 and reset

the problem is how to creat a main module and instance moudule's inside always block

note: USING xilinx fpga development board