Can I claim past student loan interest when my loan was in deferment or forbearance? I owe $18,000 for the loan, but $5,000 is in interest, can I claim the $2500?

You can only take a deduction for interest you actually paid in 2018.  

Will I ever be able to claim the $5,000.00

When you start making payments, you will get a statement from the lender of how much of each payment is interest.  You may get a monthly statement and you will get a year-end statement called a1099-E required by law.  That’s when you can start deducting interest, when you actually pay interest.