Tense, as can be observed, does not contain values for compound tenses, in general past tenses, which in corpora are not usually dealt with by automatic taggers. The possibility of re-building and deducing analytic verbal forms, by means of e.g. rules and permitted combinations of auxiliary-past participle in the various languages, is provided by values presented here as special extensions, which encode the auxiliary nature of a verb. This introduces a distinction between the requirements of an encoding scheme for corpora and for lexica: the latter also need specifications for compound tenses.
Another problem for Tense is posed by the value past, since it changes its meaning in the various verbal systems, depending on the opposition relationships among values in a language. The English past does not have the same meaning as the past in the Romance languages: it is not opposed to an imperfect value, but instead it seems to be in opposition to the present. In NERC, a proposal for the disambiguation of this complicated situation was made with the following tree:
In Romance language systems, the values past and imperfect are opposed and designate two different aspects of a past action. Both are opposed to the present with respect to the notion they represent: past is a non-durative action finished in the past and imperfect is a durative action initiated in the past. In order to avoid misunderstandings, a tentative solution could be to rename the Romance past value as perfect, since it is opposed to imperfect.