Exceptional case-marking (ECM) is a concept of the government and binding theory of syntax to analyze certain verbs. Verbs in English that have been analyzed as involving exceptional case-marking include believe and prove, as in:
This construction is traditionally referred to as accusativus cum infinitivo in the context of Latin grammar.
Under the GB analysis, the words in boldface used in the above examples, including an accusative determiner phrase (DP) and an infinitive verb phrase (VP), form a syntactic constituent, classified inflection phrase (IP). The verb is capable of granting accusative case. Therefore, the DP is analyzed as the specifier of the IPn IP-complement; otherwise the overt DP (him or her in the examples above) would not be assigned to any case, which would violate the Case Filter (which states that all overt DPs must have a case). The head I ('to') in the subordinate clause has a [-Tns] feature that is unable to assign a case. If the IP were embedded in a complementizer phrase (CP), this would certainly block case assignment; thus ECM verbs are analyzed as taking an IP-complement.
ECM verbs are often studied in relation to control verbs and raising verbs, as all three types of verbs involve relations between the argument of a verb in a main clause and the verb of what is analyzed as a non-finite clause. What makes ECM verbs different is that there is no thematic relation assigned by the verb in the main clause to the argument that receives accusative case (him and her in the examples above). Because there is no thematic relation between the main verb and the subject of the non-finite clause, the assignment of the accusative case is unexpected under GB theory. For this reason, this phenomenon is called exceptional case-marking.
Currently, theorists have mostly rejected this analysis in favor of subject-to-object raising.
ECM verbs such as "believe" or "expect" take a complementizer phrase and assign accusative case to the subject of that non-finite clause. For example, "Max expects Maria to word letters carefully." The non-finite clause is "Maria to word letters carefully." This non-finite clause is equivalent to a finite clause, as in "Max expects that Maria will word letters carefully."
In the case of -ing verbs, the use of an accusative or a genitive case pronoun can create an NP or a CP. In "I was surprised by their winning the race," "their winning the race" is an NP, while using "them" in lieu of "their" creates a CP: "them winning the race."