alignment always works with sentence tokens, 2 i.e., it relies on linear order.
this means that polysemy (one type in the SL corresponding to several types in the tL), homonymy (several types in the SL corresponding to one type in the tL), and combinations of polysemy and homonymy will disrupt the correspondence even between structurally similar languages; the circumstance that linear order cannot be used to constrain word alignment -beyond the restriction that putative word alignments must appear in one and the same sentence alignment unit-together with the other factors just mentioned, conspire to make word alignment a much harder problem than sentence alignment in the language-independent case.