Why computers do not translate better

The linguistic and computational complexities of machine translation are not always apparent to all users or potential purchasers of systems. As a consequence, they are sometimes unable to distinguish between the failings of particular systems and the problems which the best system would have. In this article I shall attempt to outline the difficulties encountered by computers in translating from one natural language into another. This is an introductory paper for those unfamiliar with what computers can and cannot achieve in this field.