Original: While there he began planning the design and construction of a suitable mausoleum and funerary garden in Agra for his wife, a task that would take more than 22 years  to complete, the Taj Mahal.
Simplified: While there he began planning the design and construction of a suitable mausoleum, a kind of tomb, and a garden in Agra for his wife, a task that would take more than 20 years to complete, the Taj Mahal.