The web-based vocabulary-learning tool IdiomsTube was launched in November 2018 to facilitate independent formulaic expression (FE) learning through YouTube videos. Using the app’s interface, a controlled experiment was conducted to: 1) compare the effectiveness of video versus reading input for second language (L2) FE learning; 2) investigate whether IdiomsTube’s automatically generated learning tasks (i.e. glossary, glosses, and cloze exercise) may enhance L2 FE learning; and 3) identify factors of successful computer-assisted L2 FE learning. Sixty-seven EFL undergraduate students took part in the experiment. Their knowledge gains were compared within subject along with information about their L2 vocabulary size, and in-app behaviour (e.g. time on task, number of clicks on hyperlinks to dictionaries). The results indicate that video and reading input are equally conducive to L2 FE learning. L2 FE knowledge gain was significantly higher when assisted by IdiomsTube’s automatically generated learning tasks (i.e. glossary, glosses, and cloze exercise) than when unassisted. Finally, logistic regression results indicate that L2 vocabulary size, time on task and looking up the dictionary are factors of FE learning in a computer-assisted language learning context.