You want to enforce $h_i > 0 \implies h_{i-1} = 8$. You can do so by introducing binary variables $x_i$ and the following constraints: \begin{align} h_i > 0 &\implies x_i = 1 \\ x_i = 1 &\implies h_{i-1} = 8 \end{align} which you can linearize as: \begin{align} h_i &\le 8 x_i \\ 8 - h_{i-1} &\le 8(1-x_i) \end{align} More compactly: $$8 x_{i+1} \le h_i \le 8 x_i$$
The exact total workload should be distributed in different days :
$$\sum_i h_i =20$$
