A sketch has dimensions and constraints. Here is how to add constraints manually:
The two red arrows show how to add a tangent constraint between a line and a circle.
The two green arrows show how to add a contraint between a WorkPoint and a line endpoint.
Inadequatrely constrained objects are black.
The bottom right status bar in sketch mode tells you whether the sketch is fully constrained or not. It will either say
Fully Contrained or N dimensions needed.