A monomer is a small molecule that can join to many others of the same kind. A polymer is a long molecule made of many monomers joined in a chain. Simple sugars such as glucose are the monomers of carbohydrates.
In synthesis, sugar monomers are joined together to make polymers. Plants make starch to store energy, animals make glycogen to store energy (they are storage molecules), and plants also make cellulose for their cell walls. These polymers are large and insoluble, so a cell can store a lot of glucose without it dissolving in the cytoplasm.
In breakdown, enzymes called carbohydrases split the polymers back into sugars. The sugars can then be used as a fuel in respiration or used to build new molecules. You can model both processes with Lego bricks: clip the bricks together to show synthesis, then pull them apart to show breakdown.
Two food tests show carbohydrates. Benedict's reagent is heated with the sample and turns brick red if sugars are present. Iodine solution turns blue-black if starch is present.