Learners use L1 knowledge to learn L2. This can be helpful, referred to as interrelatedness, or harmful, referred to as interference. L1 and L2 are related in many ways, but one of the most important connections is that a better understanding and cognitive development in L1 leads to a better acquisition of L2. Receptive language skills (listening and reading) and productive language skills (writing and speaking) have similarities across all languages, so the strength of a student in L1 is a good predictor of how well that student will acquire L2.
L1 can also negatively impact how a student acquires L2 through interference. L1 interference most commonly affects pronunciation, grammar, structures, and vocabulary.
For example, there are no irregular plural nouns in Spanish, so a native Spanish-speaker might say:
“The other childs got lost” instead of, “The other children got lost.”
Another example of Spanish interference happens because in Spanish, adjectives agree with the nouns they modify. A native Spanish-speaker might say:
“Look at these beautifuls flowers!” instead of “Look at these beautiful flowers!”
Because the noun being modified is plural, a native Spanish-speaker might also make the adjective plural.