Immersion is one of the best methods for second language acquisition. Despite its effectiveness, it is prohibitive for many people. Therefore, spaced repetition becomes a crucial tool when desiring long-term retention of sentence structure and vocabulary.
According to The Effectiveness of Computer Based Spaced Repetition In Foreign Language Instruction: A Blind Study by Evgeny Chukharev-Hudilainen and Tatiana Klepikova, using a spaced repetition system for at least 10 minutes a day, the recall of long-term vocabulary increases almost three times compared to other control groups (350).
Using a computer flashcard system is necessary for using spaced repetition correctly. Although there are many programs based on spaced repetition, only one is free and open-source: Anki
An effective flashcard system consists of both audio and visual elements. Together these elements allow for the retention of the target language. Instead of adding singular vocabulary on the face of a flashcard, rather chunks of sentences should be used.