LED Breahing DE0 nano

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example pwm_adj<= counter + 1 // that mean the increment value is "1" every clock rising edge or high

pwm_adj <= counter[25:20];how i know the increment value?

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There's no increment involved.

This simply assign to PWM_adj the same value as bits 25 to 20 of counter array

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for make easy to understand how could make it in vhdl

PWM_width <= PWM_width[5:0]+ PWM_adj;

and

PWM_adj <= counter[25:20];

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I'm not versed in VHDL. I rather use verilog.

I think you should use something like:

PWM_width <= ('0' & PWM_width(5 downto 0)) + ('0' & PWM_adj);

PWM_adj <= counter(25 downto 20);

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is it like this "pwm_width <= pwm_width[5]+ pwm_adj"?

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No. pwm_width[5] only adds a 1bit value, by taking the 5th bit from pwm_width.

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i think verilog it's quite diffucult to understand than vhdl

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People usually thinks the opposite way.

Verilog is simpler, especially for users with a C programming background.

vhdl is more powerful but syntax is somehow very different from what sw programmers are used to.

Moreover vhdl requires a strong type consistency, where verilog has looser rules.