I am buying a 98 ford escort thay say you can push start the car but will nat start just siting thay say it is the altinater but that dose not sound right what could it be

It could be the alternator. There are also a host of other things that could be the cause(s). Check the charging system to determine whether the alternator is possibly bad. A quick check is to measure battery voltage with ignition off, then again with engine running. Voltage should increase 1 - 2 volts with engine running. If voltage does not increases, then you certainly have a charging system problem.