how do plants effect rain?

Answer 1

Your apparent spelling error (replacing AFFECT with EFFECT) is not so erroneous as one might think:

Plants absorb moisture from the ground through their roots, and use that moisture to transport nutrients through the plant. They then transpire that moisture through their leaves, returning the water vapor to the atmosphere where it can condense to rain and complete the water cycle. So, plants CAN EFFECT (bring about) rain by adding moisture to the atmosphere. This is a particularly important part of the water cycle in rain forests, where the amount of direct sunlight that reaches the ground can be negligible, thus minimizing evaporation as part of the water cycle. Transpiration can be the major source of vapor return to the atmosphere in these regions.

Answer 2

Both the above answers are good, although Scott could have filled his out a bit - trees (their leaf canopies, anyway) slow down and spatter the rain as it falls, and the dead leaf and other organic debris under the trees act to slow and scatter the water runoff before it reaches the soil. The end result is to really cut back on soil erosion wherever undisturbed woodlands are.

Answer 3

they stop it from hitting the ground