This study involved students from a teacher training programme and classroom teachers during in-service training. These students and teachers were introduced to variation theory and carried out a learn-ing study on English as a Second Language (ESL) that incorporated five research lessons taught in paral-lel, rather than in a cycle. The participants in the study were five classes from grade level five to upper secondary school, five university students, and two researchers. The aim was to put learning study to test in describing in what ways students (from fifth graders to upper secondary school students) discerned the letter s at the end of a word, and secondly what kind of knowledge about this learning object they were able to develop during instruction. When an s appears as a terminal letter in English, it can be inter-preted in at least five different ways: contraction, plural, third person singular, genitive or possessive pronoun. It can also be the final letter of a monomorphemic word (bass) or suffix (-ness). Our study dem-onstrated how learning study was used to describe how students of different ages interpret the suffix s. A pattern emerged indicating the way knowledge of a phenomenon develops as a consequence of teaching. This pattern was analysed in terms of the structure of the students’ native language. The outcome showed how students tried to comprehend a second language by means of the structure of the first. A good exam-ple is the pronoun your (dependent possessive form) and yours (independent possessive form). As there is no variation in Swedish between dependent and independent possessives, students associate the two forms with the differences between d- and t- gender. This distinction is made in Swedish (din/ditt) but not in modern English.