When you are creating a garment you need to marry the fabric created with a silhouette that works to compliment, not fight the fabric. How do you best learn to do this you ask? It's a lesson we learn by experience and especially by making mistakes. A very heavy detailed fabric won’t be flattering or comfortable in a snug, figure hugging shape. A swing shaped over-sized cardigan needs a light flowing fabric or it will hang heavily in folds which we may or may not want. It may make the wearer look heavy by standing away from the body and obscuring your natural contours.
When choosing patterns look carefully at the details and remember you are looking at a photo. You can’t touch the fabric so its properties are a mystery to you. Assess your swatch carefully so that you understand how the fabric relates to the silhouette, especially if you have substituted yarn. The relationship of these two factors is not one that is easily determined in advance. This is why garment makers reuse fabrications and silhouettes that they are familiar with and utilize their past experience with those mediums. To accelerate your understanding of these relationships look at garments in your closet, what other knitters are making and at retail clothing.
“Good judgment comes from experience and experience comes from bad judgment.” Fred Brooks