Underlying form and surface form are often used in the Optimality theory to explain phenomena in phonology and morphophonology.
Underlying form is the abstract form that a word, morfeem or phoneme in the mental grammar of the language user is assumed to mean before it changes as a result of one or more phonological rules. It follows that alloys and allomorphs that change as a result of alternation have the same underlying shape. The idea is that the language of the language of the language user in question also has an abstract underlying form, which can not, however, "come out".
Surface form is the term for the form of a word, morphem or phoneme as it appears after the application of certain phonological rules. Examples
In Dutch, the final sounds of the words are good and distress, which in their surface shape as a result of final sound sharpening sound the same as the end sounds in wood and error. According to the OT, the first two words have different end-sounds in their underlying form, namely voiced or voiceless dental.
Another example is the output that marks the past times of weak verbs. Depending on the final sound of the trunk of the verb, this output is -te or -de. The Optimal Theory assumes that there is only one underlying form for these two outputs, which changes as a result of the application of progressive assimilation. In the OT you could translate this as a constraint: M: Agree (Voi). Also see
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