Subtodo is a function of the pro version, you will find it after upgrade to the pro version.

Web

Move your mouse on the todo and you will find the > button on the left. Click this button to view the subtask feature.

Mac/Windows Version

Move your mouse on the todo and you will find the … button on the right. Click this button to enter the todo editing view. In this page, you can view or edit your subtodos.

Android/iOS Version

Click the todo description to enter the todo editing view. In this page, ​you can edit your subtodos.