BESIII has collected 2.93 fb −1 and 6.32 fb −1 of 𝑒 + 𝑒 − collision data samples at the center-of-mass energies 3.773 and 4.178-4.226 GeV, respectively. We report recent measurements of the (semi-) leptonic decays 𝜏) and 𝐷 (𝑠) → 𝑋ℓ + 𝜈 ℓ [𝑋 = 𝜂, 𝐾 1 (1270), 𝜌 or denotes generic decays, and ℓ = 𝑒, 𝜇]. The decay constant 𝑓 𝐷 + 𝑠 , the semileptonic form factor 𝑓 𝜂 + (0) and the CKM matrix elements |𝑉 𝑐𝑑 (𝑠) | are determined precisely. These results are important to verify the theoretical

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... ns of 𝑓 𝐷 + 𝑠 and 𝑓 𝜂 + (0), and the unitarity of the CKM matrix. Precision tests of lepton-flavor universality with (semi-) leptonic 𝐷 (𝑠) decays are also performed.