How can you convert a pal DVD to ntsc?

PAL is the European color encoding system while NTSC is the North American system. If material has been encoded in PAL it is also (almost always) using a 50Hz field rate with 576 lines making up the image while NTSC will be 60Hz with 480 lines. To perform a full conversion, a new signal must be generated from the original. The field rate needs to be changed and the number of lines also needs to be changed. Both are complex processes requiring some powerful computing. Broadcasters use the technique on a regular basis whenever content is created in one system and broadcast in the other. Typically, they will spend $50000 on a standards converter to do the job. Domestic equivalents are available at a much lower cost but the conversion quality normally leaves something to be desired. Despite the lower quality, domestic standards converters will still run into thousands of dollars. If a full conversion needs to be done, there are post production houses that can carry out the conversion at a fraction of the cost of the converter. Most will only convert material when they are sure that copyright will not be infringed by doing so. If it's a commercial DVD, most will refuse to convert or copy it.
All is not lost though. Most modern DVD players and televisions will handle both standards. Playing a PAL DVD on North American equipment is possible if you are lucky enough to have compatible equipment. In Europe, it is even more likely that the equipment will be multi-standard. Once again, there is a potential problem: Commercial DVDs are often limited to certain regions. A DVD coded for use in North America will not be playable in other parts of the world. The limitation is there to enforce distribution agreements. Despite having multi-standard equipment, the regional coding may stop you playing the DVD. This coding does not apply to home produced DVDs so it's well worth trying if the DVD is your won creation.