Going to the official source, see IRS Publication 970 (2018 tax year link). The point of contention will be the official definition of "obligated". Per Publication 970, it has this to say:
Don't Include as Interest
You can't claim a student loan interest deduction for any of the
following items.
- Interest you paid on a loan if, under the terms of the loan, you aren't legally obligated to make interest payments.
What does "legally obligated" mean? They don't say further, however in the next section they do say this (emphasis mine):
When Must Interest Be Paid?
You can deduct all interest you paid during the year on your student
loan, including voluntary payments, until the loan is paid off.
Because of this juxtaposition, I would argue the following: you are legally obligated to make interest payments (eventually, even if not starting right now due to forbearance/grace periods); starting payments now is voluntary, in the same way that paying interest in advance is also voluntary and would seemingly be permitted as well. So that would suggest you are entitled to claim the deduction if you paid the interest during that tax year.
Disclaimer: I'm neither the IRS nor a lawyer, so if they disagree with this interpretation then you'll be the one dealing with them, and I give no meaningful guarantees. As always, you should read the official sources yourself, especially explanatory publications, and use your own judgement. :)
Some random debt company website I found supports my interpretation, for whatever that is worth (nothing, but I include it as proof that at least one other person on the planet reads things the same way I do). They say:
If you are on a repayment plan that results in a lower monthly payment
than interest accruing each month or your loans are in deferment,
forbearance, or the loan has not yet entered repayment status, you can
deduct extra or voluntary payments up to the extent they are allocated
as interest for tax purposes. For example, if you are on the income
based repayment plan (IBR) with a $10 per month payment but the
interest is $150 per month and you pay an extra $100 then you may be
able to include this extra payment if you qualify for the tax
deduction.
As for a 1098-E, according to the IRS publication:
Form 1098-E. To help you figure your student loan interest deduction, you should receive Form 1098-E, Student Loan Interest
Statement. Generally, an institution (such as a bank or
governmental agency) that received interest payments of $600 or
more during 2018 on one or more qualified student loans must send Form
1098-E (or an acceptable substitute) to each borrower by January
31, 2019.
So don't expect it if you didn't pay at least $600 in interest, and they are supposed to send it by the end of January each year. I generally don't suggest anyone with non-trivial taxes submit their taxes before mid-February, but that's just my policy to avoid having to do amendments (I've always had all forms by the first of March, YMMV).
As a final note, remember that student loan interest acts to reduce taxable income, so $600 in deductions will generally equal much less than a $600 change in taxes. There is also a phaseout starting at as little as $65,000 modified adjusted gross income income, but there is no minimum.
And as a final suggestion, I always recommend with items like this that you use a good tax software and that you try entering all your information, and then alternatively add/remove anything optional to see how it effects your bottom-line amount owed/refunded. I've had situations where entering a deduction actually made my taxes worse, because I initially misunderstood it, the tax software handled it wrong (a $9000+ near-loss made me not use TurboTax ever again), because there is more than one valid interpretation of how to account for something, or because tax regulations are so Byzantine by design that some benefits can reduce or eliminate other benefits in an amount greater than the benefit claimed (because of course that could happen, why not?).
