The command line instructions are very good in CAD programs, the ones that support them that is.
Once you start getting up to speed it's well worth printing out the command line structure for common moves.
Type in "L" [ no inverted commas] in the command line and it knows it has to draw a line, first question is start point ? Type in 0,0 and it will start at the drawing base point. Then it wants the next point.
As most drawings are dimensioned as a part and not from a common point we are used to so wide, so long as opposed to always measuring from an absolute point like 0,0. From an absolute point is called , well absolute and the so wide, so long method is called Relative.
So to carry on with our line if we type in 100,0 we get a line 100 units long none high so parallel, now if we need a vertical side 30mm high we type in 100,30
This side is Absolute to the start point and can be confusing if you have a sketch that doesn't give any idea of where every point is to the absolute start point, even harder is the absolute point isn't at 0,0
The choice is now to work in relative which adds entities on from the last point and doesn't care where the start point is.
On the command line we have alternative moves that are highlighted in blue, Undo is one as it's a likely next move, so press U then enter and the vertical line disappears, press U enter again and the horizontal line disappears.
Now type in @100,0 and we get the same horizontal move as before because it starting from the same point, however now type in @0,30 and we get the same vertical line 30 units long but starting from the last point.
@50,50 will get us a line 50 units along it's base and 50 units high, in other words a line at 45 degrees but we don't know how long that line is without using trig.
But if we type @50<45 it now draws a line 50 units long at 45 degrees.
Here endeth this lesson.
@ gets you relative moves from the last point.
< gets you angular moves.
U enter gets you an undo move.