Old English had only two tenses: present and past. An alternative to the many inflectional forms that other languages have is representing the tenses and moods by adding auxiliary verbs. Spanish uses this approach with the progressive and perfect tenses. Traditionally, modals are considered defective auxiliary verbs that indicate certain conditions such as permission, necessity, obligation, suggestion, and prohibition. The future auxiliary (not the modal auxiliary) 'will' has no distinct Spanish counterpart either: Spanish simply has a future conjugation. Another modal verb that we see mostly in older writing and very formal contexts is 'need', as in "With her great talents, Luz need not graduate to find a good job." A quote from grammar.about.com:
"The most important syntactic developments which distinguish [modals] from other verbs are the following:
(1) they lost their non-finite forms and their ability to take non-verbal objects;
(2) the preterite forms came to be used in the present, future or timeless contexts;
(3) they did not develop the to- link with an infinitive (in the Southern standard);
(4) they became more and more uncommon in contexts where they were not followed by an infinitive."
(Richard M. Hogg, et al., The Cambridge History of the English Language: 1476-1776. Cambridge Univ. Press, 1999)