In this chapter we will consider some particular problems which the
task of translation poses for the builder of MT systems --- some of
the reasons why MT is hard. It is useful to think of these problems
under two headings: (i) Problems of ambiguity , (ii) problems that
arise from structural and lexical differences between
languages and (iii) multiword units like idiom s and collocations . We
will discuss typical problems of ambiguity in Section 
,
lexical and structural mismatches in Section , and
multiword units in Section .
Of course, these sorts of problem are not the only reasons why MT is hard. Other problems include the sheer size of the undertaking, as indicated by the number of rules and dictionary entries that a realistic system will need, and the fact that there are many constructions whose grammar is poorly understood, in the sense that it is not clear how they should be represented, or what rules should be used to describe them. This is the case even for English, which has been extensively studied, and for which there are detailed descriptions -- both traditional `descriptive' and theoretically sophisticated -- some of which are written with computational usability in mind. It is an even worse problem for other languages. Moreover, even where there is a reasonable description of a phenomenon or construction, producing a description which is sufficiently precise to be used by an automatic system raises non-trivial problems.