Mostly, a pattern asks for a yarn that is not available. A substitute must come. But how do I replace it correctly? Not only the yarn weight is important, but also the fiber content. Each fiber has different attributs and can affect the appearance of the knitted fabric. I'll show you the characteristics of the fibers.
How does the knitted garment looks like on the pictures of your pattern? Should it be soft and drapey? Does it shine or is it dull and hairy? If it has texture, twisted stitches are knitted or are there cables? These stitches are more difficult to knit with non-elastic yarn such as cotton or linen. Is there a textured pattern with right and left stitches? This wouldn't be noticeable with a hairy or fluffy yarn.
To replace the yarn from a knitting or crochet pattern, you need to know not only the yarn weight, but also the fiber content. Each fiber has different properties. The following list of fibers is intended to give you a clue as to how the knitted fabric looks and what character characteristics the fibers have.
With which fiber do you prefer to knit with? And why?