You can use IntelliSketch to connect an element you are drawing with an existing element. You can apply a connect relationship as you draw the lines, or draw the line without a connect relationship.
Choose the IntelliSketch command 
.
In the IntelliSketch dialog box, on the Relationships tab, set the End Point option, and then click OK.
Choose the Line command.
Move the cursor to the end of a line in the application window. As you move the cursor over it, the line is highlighted and IntelliSketch displays the End Point relationship indicator at the cursor.
While IntelliSketch displays the relationship indicator, click to place the first end point of the new line (A). This end point is connected to the end point of the previous line.
Tip:
Rather than clicking, you can snap to the line end point nearest the cursor by pressing the E key.
Click where you want to place the second end point of the new line.
The new line and the previous line have connected end points (B).
IntelliSketch places a connect relationship handle at the point where the two lines connect (C).
Tip:
Relationship handles can be displayed or hidden with the Relationship Handles command.
Tip:
Relationships are maintained only if the Maintain Relationships command is set.
Tip:
To snap to a keypoint or intersection point, locate the element(s) with the cursor and then press one of these shortcut keys.
Midpoint of a line or arc: press M.
Intersection point of lines, circle, curves, and arcs: press I.
Center point of a circle or arc: press C.
Endpoint of a line, arc, or curve: press E.
For intersection points--If there are multiple eligible points located, then QuickPick opens and lists them. In QuickPick, click to select the point you want.