The stochastic process $\{X(t), t \geq 0\}$ is called homogeneous and will have stationary jump probability if $$P_{ij}(s, t)=P(x(t)=j \mid x(s)=i))=P_{ij}(0, t-s), \forall t, s \geq 0$$
We write $P_{ij}(t-s)$ for the probabilities $P_{ij}(s, t)$.
The Chapman-Kolmogorov equations are written as $$P_{ij}(t+s)=\sum_{k=0}^\infty P_{kj}(t)P_{kj}(s), \forall t, s \geq 0$$
Can you explain to me why at the Chapman-Kolmogorov equations we have a formula for $P_{ij}(t+s)$ and before we had $P_{ij}(t-s)$ ???
Why does it stand that at the Chapman-Kolmogorov equations at $P_{kj}(t)$ the index is $kj$ and not $ik$ ???