A study of one of the most recurrent and persistent obstacles in learning English as a second language, the use of tense marking, is presented. The analysis is based on audio recordings from a Vietnamese community in Northern Virginia, representing four age ranges from 10 to 55 years, two lengths of U.S. residency (1-3 years and 4-7 years), and both males and females. A range of possible effects on variable tense marking patterns is considered, including surface form distinctions such as regular and irregular forms, phonological shape of those forms, the linguistic context of the forms, lexical items, and relative frequency of the forms. Higher order effects such as foregrounding, episodic boundaries, and tense continuity within narratives are also considered in both spoken and written language. The analysis reveals that surface-level constraints are more significant than the higher-order constraints, and that several principles account for the systematic relationships of variability among forms. Written language constraints show both similarities and differences with spoken language, demonstrating an important lack of isomorphy in the two language registers. The nature of tense marking in interlanguage is shown to have some corollaries with the way tense marking has been treated traditionally in formal instruction. It is concluded, however, that this similarity derives from some general principles of second language acquisition rather than the primary influence of formal instruction on English tense formation. (Author/MSE)