Two figures are tangents when they have only one point in common, the point of tangency (T).
The most common tangencies can occur among circumference ands traight lines or among circumferences.
In order to draw precise tangencies, learn the followings theorems:
First theorem: a straight line is tangent to a circumference when both have only one point in common. The radius of the circumference at the point of tangency is at right angle to the tangent.
Second theorem: a circumference is tangencial to two straight lines from a common pointi fits centre is on the angle bisector of the angle formed by the straight lines.