This paper analyzes the spoken corpus of thirty Iranian learners of French at four levels (A1, A2, B1 and B2). The data were collected in a pseudo-longitudinal manner in semi-directed interviews with half closed and open questions to analyze the learners' syntactic errors (omission, addition, substitution and displacement). The most frequent errors from A1 to B2 levels were determined and categorized, which allowed further analysis. The results show although some persistent errors are problematic at all levels, their form changes from one level to another. At A1 and A2 levels, the definite articles were missed before nouns, at B1 and B2, they were set aside in the second word of noun groups. In addition, at A1 and A2 levels errors are more of an interlingual nature while at B1 and B2 levels they are mostly intralingual. At elementary levels, learners deal with proactive transfers whereas in more advanced levels retroactive transfers and overextensions come into play.