Halliday & Matthiessen (1999: 26):
In traditional grammar, only certain grammatical categories were taken into consideration; these categories were (i) overt and (ii) word-based. In particular, inflectional categories of the word such as tense, case and number were described and then interpreted semantically. In a functional grammar, while such categories are not ignored, they tend to play a less significant rôle, appearing at the end point of realisational chains. For instance, it is not possible to base a functional interpretation of number in English simply on the presence or absence of ‘plural’ as a nominal suffix (as in grammar+s); the category of number is more complex, involving two complementary systems. Similarly, the general properties of of the construal of time in the English tense system are not revealed by only looking at the overt suffixial past tense marker (as in laugh+ed); again the scope of the semantics of tense in English is far greater than this overt word category would suggest.