I've tried proportional editing but it is ineffective and very unsmooth. A cloth simulation with pinned corners doesn't work either. I am relatively new to blender and would appreciate the help :) Thank you!
The cloth solution referenced by @batFINGER works for me:
Starting with the two (nearly) straight edges, first, Bridge Edge Loops with settings something like these:
.. and you're nearly there. A bit of Vertex > Smooth-ing on the internal vertices might do. But if you want to, you can finish the stretch with a quick Cloth sim, as shown on the right, above, with settings very similar to those provided as 'Tent' in the referenced answer, and the straight edges pinned throughout, rather than just the corners.
(the sim runs to stability in about 30 frames, in the provided .blend)
I assume that you have used a big plane with 4 verticies. Subdivide the plane and use proportional editing (press the letter O on the keyboard). This would be a possibility, otherwise you could also use a lattice modifier on the square with the corresponding lattice. But keep in mind, so or so you have to subdivide the square.
The lattice approach:
after the steps, move the lattice verticies around in edit mode
As Jack Pillawa says you can do it with a Lattice modifier:
- Subdivide your plane, give it a Subdivision Surface modifier:
- Create a lattice (ShiftA), give it a Resolution of 3/3/1, scale it in Object mode so that it fits the plane size:
- Give your plane a Lattice modifier with the lattice as Object. Twist the lattice in order to deform the plane. You can create shapekeys for your lattice if you want to keep its basic flat shape: