On the top row of example 1 there are 4 more circles left. However, a maximum of 1 check mark can be placed on the back 2 circles. That means there must be check marks on the first 2 circles.
On the 3rd row you see 3 circles next to each other. A maximum of 2 check marks can be placed on these 3 circles. So you can fill the 3rd row completely.
It is useful to put a "cross" yourself above, below and next to check marks already placed.