Compound words with and without hyphen

Hi all.

I am confused in using the hyphen in compound words, I understand that we use
a hyphen with compound words with two or more syllables, and we can drop it with words with one syllable. But, I do see many words not adhere with tis rule.
Please anyone can explain this to me; what is the rule of using the hyphen in compund words ??, and when can I drop it?.
I have some for example: eyewitness or eye-witness. Fiisrthand or first-hand.
Giveaway or give-away. kickoff or kick-off.

Thanks in advance