We consider suspension flows over uniquely ergodic skew-translations on a d-dimensional torus T d , for d ≥ 2. We prove that there exists a set R of smooth functions, which is dense in the space C (T d ) of continuous functions, such that every roof function in R which is not cohomologous to a constant induces a mixing suspension flow. We also construct a dense set of mixing examples which is explicitly described in terms of their Fourier coefficients. In the language of nilflows on nilmanifolds, our result implies that, for every uniquely ergodic nilflow on a quasi-abelian filiform nilmanifold, there exists a dense subspace of smooth time-changes in which mixing occurs if and only if the time-change is not cohomologous to a constant. This generalizes a theorem by Avila, Forni and Ulcigrai (J. Diff. Geom., 2011) for the classical Heisenberg group.In this section, we present the general structure of the proof of Theorem 1.1-(a), stating some intermediate results, which are proved in later sections.Let T be a uniquely ergodic skew-translation as in (1.1). If we denote by E j the image of the linear map (A − Id) j , we have a filtration of R d into rational subspacesUp to a linear isomorphism, we can assume that the basis {e 1 , . . . , e d } of Z d is adapted to the filtration above, in particular {e d0+1 , . . . , e d } is a basis of E k , where d − d 0 = dim E k . Since T is not a rotation, k ≥ 1 and 1 ≤ d 0 ≤ d − 1. We remark that w ∈ E j for j ≥ 1 if and only if there exists v ∈ E j−1 such that v(A − Id) = w, i.e. vA = v + w. In particular,