Negative transfer, also called interference or L1 interference, occurs when properties of the learner's first language produce errors in the second. A Vietnamese learner saying I have 25 years under the influence of Tôi có 25 tuổi, or a French speaker pronouncing English think with a fronted /s/-like fricative because French lacks /θ/, is experiencing negative transfer at the syntactic and phonological levels respectively. The term is half of the positive/negative pair introduced by Lado (1957) within the Contrastive Analysis Hypothesis.
Lado argued that L2 learning difficulty is a function of L1-L2 distance: similar features facilitate, different features interfere. The strong version of the hypothesis predicted that all L2 errors could be traced to L1 interference and that systematic comparison of the two languages would yield exhaustive lists of predicted errors. Empirical work in the 1970s undermined this. Many predicted errors did not appear, many actual errors had nothing to do with L1, and some errors were uniform across L1 backgrounds (developmental rather than transfer-driven).
The concept survived the demise of the strong hypothesis and was reframed under the broader umbrella of crosslinguistic influence (Kellerman and Sharwood Smith 1986; Odlin 1989). Negative transfer is now understood as one outcome among many rather than the master explanation of L2 errors.
Following Jarvis and Pavlenko (2008), negative transfer operates at every linguistic level:
Transfer is also possible as avoidance: learners underuse L2 structures that differ markedly from L1 equivalents, which Schachter (1974) showed in Chinese and Japanese learners producing fewer English relative clauses than expected for their proficiency.
Not every L1-L2 difference produces transfer, and the same difference produces transfer in some learners but not others. Key variables:
For Vietnamese English learners, predictable negative transfer includes the absence of inflectional morphology (third-person -s, plural -s, past -ed) under the influence of Vietnamese isolating morphology; difficulty with consonant clusters and final consonants; substitution of /θ/ and /ð/ with /t/ and /d/; transferred topic-comment word order; calqued idioms (open the television); and tense-aspect mapping mismatches given Vietnamese aspect-marker system.
Useful teaching responses combine explicit contrastive awareness, focused noticing tasks, and corrective feedback that targets recurring transfer patterns rather than treating them as one-off errors. Negative transfer often becomes fossilised when uncorrected at a stage where it stops impeding communication, so addressing it earlier is generally more effective than later.