You are talking about "pre-tax" and "post-tax" but those terms are imprecise and giving you confusion.
There are two things you need to worry about.
The first is whether you are eligible for a deduction on traditional IRA contributions. As mlejw6 points out, you can claim a deduction on your tax return for contributions to a traditional IRA if your modified adjusted gross income falls below a certain threshold. See this for more information:
https://www.irs.gov/Retirement-Plans/IRA-Deduction-Limits"But," you ask, "what good is a deduction if my employer withholds the same amount from my paycheck each pay period?"
That is a good question and brings us to the second thing to worry about: how much your employer withholds from your paycheck each pay period to pay to the government.
Most people don't realize it, but you control how much your employer withholds from your check every month. You tell the employer by giving the employer a Form W-4 that specifies a certain number of withholding allowances. Most people sign a W-4 when they are hired (without understanding what they are signing) and never think about it again. But the W-4 is an important tool for you to learn how to use to make sure you don't have too much money withheld from your paycheck each month. Adjusting your withholding could free up extra cash, making it easier for you to put money into your traditional IRA.
One of the other forum members, MDM, recently walked me through how to understanding the relationship between allowances claimed on a Form W-4 and the amount withheld from your paycheck. Check out this thread for more information:
http://forum.mrmoneymustache.com/taxes/best-way-to-calculate-w-4-exemptions-for-2016/Hope this helps.
Edit to add: One word of caution. You also should not have too little withheld from your paycheck. If you do, you may have to pay a penalty. See:
https://www.irs.gov/taxtopics/tc306.htmlWelcome to the forum!
