“Direct sunlight is sun shining right on the plant at least 6 hours a day, not through a window,” Renee Shepherd of Renee’s Garden graciously explained. This means if you put your hand over the plant and sunlight hits your hand, that’s direct sunlight. “Shade is literally shady, like under an umbrella, but partial or dappled shade depends on the plant,” she says. “Partial shade can mean morning sun and afternoon shade, or morning shade and afternoon sun, which means sunshine in the afternoon will be hotter.” As a good rule of thumb, dappled shade is when the sun gets filtered through a trellis or through leaves.
“When considering which crops to grow in shady areas,” Mother Earth News puts succinctly, “Think of them in terms of leaves and roots. Crops we grow for their leaves (kale, lettuce, spinach) and those we grow for their roots (beets, carrots, turnips) will do fairly well in partially shady conditions. The crops we grow for their fruits — such as eggplants, peppers and tomatoes — really do need at least six hours of full sun per day.”
A good quick way to see if your plants aren’t getting enough sun is if you see them bending toward a light source, if they’re growth is especially slow, or if the plants look “leggy,” meaning they look kind of spindly or elongated.
There’s lots of good information here, especially when it comes to heat vs. sunlight. Living in Los Angeles, plants that a person in Minnesota would grow in the open, I have to put in light shade just because the heat of summer can damage the plant. So, please pay attention to your climate zones. If you’re in the US, you can use the USDA’s Zone Map. Other countries have similar maps and your state’s extension office can help you with more detail.