i tagged tasks with an name of a project. Now this project is so big and so i created an new project. I want to move the closed tasks to this new project.
unfortunately, it's not possible to move completed tasks between projects, but we will consider adding such feature in the future. You're welcome to add a request to our Votebox: http://todoist.com/Vote/
For now, as a workaround, you could click on the "Get completed" link at the bottom of your task list, click on the checkbox to un-check them, then move them to a different project using the multi-select feature (hold down CTRL and then click on each task) and check them off within the target project.
Alternatively, you could archive the old project by right-clicking on its name and choosing "Archive" from the context menu and then just continue within the new project. All completed tasks will remain within the archived project.