Does it matter if we use saç or saçlar?
both are acceptable but it is often used in plural in Turkish
why I can't use this expression with "Saçların-ı"?
Subjects of sentences will never be in the accusative case. Never ever.
Why we have to use " senin saçların" while "senin saçlar" is not accepted?
You need to use the possessive ending in the noun. "Senin" is optional; the "ın" in "saçların" is not.