Can you knit flat with circular needles? Yes! You absolutely can.
Yes you can! Do not let anyone tell you it's impossible unless they are willing to demonstrate why. I've seen some bonkers comments regarding circulars in the past. "You can't use circulars because you have to keep flipping your work" no, no you don't. You finish your row and start working back on the other needle, just like with straight knitting pins.
Circular needles are used for knitting in the round, however, they are perfectly handy used as flat needles too. Instead of the long, straight needles, you have a long cord. This cord takes an incredible amount of weight off your wrists if you're making bigger items such as adult garments. You also have the added bonus of length. Straight needles go up to 40cm long, which isn't long enough if you want to cast on an entire blanket or a one piece item such as a top down cardigan. Circular knitting cords come in many lengths meaning the sky is the limit!
If you're not an armpit knitter, you may also find that circulars allow you to work in tighter spaces such as on public transport or a high sided chair. There's reduced risk of knocking into anything or anyone, which often results in dropped stitches.
When you're finished knitting with your circular needles, make sure you place some needle stoppers on the tips. This stops anyone from being impaled but more importantly, your work won't slide off.
Drawbacks of knitting flat with circulars
There are only some, so watch out for them:
1. They're incredibly light so you could have trouble casting on.
Keep hold of them and keep an eye on them. Perhaps lay the needles on your lap or on a hard surface to make sure they don't fall onto the floor.
2. Sliding on to the cable.
Stitches will slide on to the cable and this is where you need to manage your tension. If you're casting on too tightly, those stitches won't slide back up the needle so that you can knit with them! try and keep the stitches even and wide enough to fit onto the needles.
3. Dropping stitches.
Dropping stitches is sometimes really hard to avoid. I actually tend to drop a lot of stitches on straight needles as I seem to knock into them a lot or get them caught in my sleeve. With circulars, you could drop the stitches simply because the circular cord has been weighed down too much by your work. This will then pull the needle downwards and out of the stitches! Keep an eye on it and work with your lap or a hard surface when you are starting out.