Novel liquid-crystal tunable metamaterial devices for terahertz wave manipulation are designed and investigated. Deeply subwavelength layers of nematic materials are introduced in metal-liquid-crystal-metal cavities, leading to strong interaction of the THz wave with the liquid crystal anisotropy. The enhanced response in the liquid-crystal layer yields extensive tunability of the electromagnetic resonances with fast response times in the millisecond range. Two specific components are targeted: a tunable metamaterial absorber as amplitude modulator and a LC-tunable polarization converter.