Of course, some errors may be due to “transfer” (also called “crosslinguistic influence”).
Transfer means using sounds, expressions or structures from the L1 when
performing in the L2. For example, a Spanish (L1) speaker who produces take it from
the side inferior may be trying to use the Spanish adjective inferior (= lower in English)
and placing it after the noun, as is typical in Spanish constructions. If the L1 and L2
have similar features (e.g. marking plural on the ends of nouns), then the learner may
be able to benefit from the positive transfer of L1 knowledge to the L2. On the other
hand, transferring an L1 feature that is really different from the L2 (e.g. putting the
adjective after the noun) results in negative transfer and it may make the L2 expression
difficult to understand. We should remember that negative transfer (sometimes
called “interference”) is more common in the early stages of L2 learning and often
decreases as the learner develops familiarity with the L2.