Spanish has a large number of irregular verbs.
Luckily, most of them can be categorized in groups which are irregular in the same way.
In this lesson you will learn a group of verbs that changes the first person ending -o into -go in the present tense.
Common verbs which do this are hacer, salir, valer and poner.
If another verb ends in one of these verbs, for example suponer, you can be almost certain that it will conjugate in the same way.
|to do, to make
|to leave, to go out
|to be worth, to cost
|-o → -go
Note that hacer is an exception here, since we don't only add a g, but also drop the c to make hago.