This paper analyses the use of periphrastic "do" in negative declaratives and non-negative questions with inversion by 200 Swiss and 30 German middle-school students of the same age but with different onsets of learning English and consequently a different amount of foreign language instructions. "Do"-support shows evidence of a variety of influences on the learning of a foreign language in an instructional setting, from cross-linguistic influences to the role of the typological difference among the languages involved and the kind and length of instructions. By means of a fine-grained characterisation of the learners' negation and interrogation patterns, it will be shown that there are important differences in the way multilingual learners with differing ages of onset and different L1s, L2s and L3s create their own rule systems in the process of learning the use of periphrastic "do" in L3/L4 English. Most importantly, the learners' productive knowledge and grammaticality judgments cannot be fully explained in terms of either the structure of their L1 (Swiss vs. Standard German) or of the target language (English), which indicates the incorporation of L2 (French) in their L3 utterances and grammaticality judgments and thus supports the hypothesis of syntactic transfer from L2 to L3.