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Sometimes, you may be asked to file an new task/issue or subtask, based on a conversation you had on a mailing list or you may want to split your task into several smaller tasks (subtasks) to organize things a bit.
To create a new issue, take a look at this page. To create a subtask from the Work view, click on a task, and in the side panel, go to the button on the upper right corner (...) > Create Sub-Task:
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This will bring up a form similar to the Create Issue form described in the aforementioned link that can be filled out.
Lastly...
If you are working on a task, and are stuck, don't be afraid to ask questions - it is in everyone's interest to move the project forward. However, please be sure to search existing documentations and mailing list archives before doing so, or the response you get may consist of a link to a page or something similarly terse.