In modern English: if you qualify to be claimed as the dependent of another taxpayer, you may NOT claim a personal exemption for yourself, nor may you claim any dependents of your own on your tax return.
No ifs, ands, or buts: as long as the other taxpayer(s) COULD have claimed you on their tax return, it does not matter whether they actually DID claim you on their tax return--you may not claim yourself on your return nor may you claim any dependents on your return.
For more information about rules for personal and dependent exemptions, see here.
There are some other negatives for taxpayers who can be claimed as dependents. In particular, if you can be claimed as a dependent by another taxpayer, you are NOT eligible for the Making Work Pay refundable tax credit.
This led to some unpleasant surprises for young dependent taxpayers last year. Some of them ended up owing money, when they had been expecting at least a small refund. That's because payroll withholding formulas did not take into account their dependent status, so they had not had enough federal tax withheld from their pay.