Dependency

"Build starts when design is done" describes a dependency. So does "QA comes after build." Record those relationships as you describe the work, then draw them as arrows between the tasks.

The most common relationship is finish-to-start: A finishes, then B starts. Including that link lets you work out how a delay in A affects B. Without it, the dates may look feasible even when the work can't happen in that order.

Avoid linking tasks merely because you'd prefer them in a certain order. Extra links can make a plan harder to update and create delays that the work doesn't require. Check what actually prevents each task from proceeding.