Question

hey! listen I'am unable to understand when to place + or - sign in the lens formulae

Answer 1

The formula is always 

        1/v - 1/u = 1/f
          magnification is  m = v/u = h'/h

Sign convention:

1.  Distances  u , v ,  f   measured against the direction of  rays from object are negative.  If object on the left, lens on the right side, then  u is negative.  Focal length for convex lens is +ve.  Focal length for  concave lens is negative.

2.  Distances or heights h or h' of objects and images which are erect (+Y direction) are positive. Inverted lengths h' or h  in -y direction are negative. 

Substitute +ve or negative sign along with numerical values.  

One good thing, you can do,  call  U, V, F, H, H' to be always positive and equal to |u|, | v | ,....

Then for a concave lens,  u = - U,  v = - V, f = -F.   h = H,  h' = H'
For a convex lens for real image,  h = H,  h' = -H'   f = F,  u = -U, v = -V.

Make a table of  all signs for all virtual and real images for all lenses. Then you will understand it.