Answer: Mandevilla is a tropical vine that blooms on new growth, and the new vine can normally look a bit thin or whippy until it fills out. If you just purchased the plant, it probably does not need pruning.
If you kept the plant indoors over the winter, then it might need to be trimmed back if it has grown very thin and weak and/or lost some leaves. You could trim it back quite hard, by half or two thirds to stimulate new growth. If it has died back on its own, trim the dead portion off.
Gradually acclimate it to being outdoors in a bright sunny location for the summer, and do not leave it outside on cold nights (under about 50 to 55 degrees). Fertilize it regularly with a complete water soluble houseplant fertilizer (read and follow the directions) and water often enough to keep the soil evenly moist. Once it is growing vigorously again and temperatures are warm it should begin blooming again.
Enjoy your mandevilla!
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