Maybe. The answer depends on too many variables for anyone to answer it here. If you have other income (such as income from a spouse), then yes, a portion of your benefits may be taxable. If you are single and had no other income, then the benefits may not be taxable. You should consult an experienced CPA or other tax preparere for a review of your situation and help preparing the return, if one is needed.
This answer is not intended to provide you with specific legal advice regarding your situation, or to create any attorney-client relationship. The intent is only to provide general information. You should be aware that you cannot rely on this answer to provide you with any protection against tax penalties. You should always consult your own attorney in order to obtain legal advice.
Yes the monthly SSA benefits are considered taxable income to the extent they replace lost income. Whether you will actually owe tax on the taxable income depends on your relative amounts of income and deductions.
It is taxable, but lump sum payments can be applied to multiple years using the "LSE Method." Speak with a tax preparer about doing this and it will not be lumped into one year and taxed at a higher rate as a result.