My daughter will be two years old in 4 months, and she still doesn't like to chew her food. She won't spit it out; she just tries to swallow it whole.
What should I do to help her learn to chew her food?
Your daughter is about the same age as my second son (also 20 months), and he is a very interesting case in this regard. He's demanded food that could be chewed since, no joke, 4 months old; no mash for him, only at least vaguely toothy food. But, he often doesn't chew still to this day - mashes it around and then tries to swallow, but doesn't properly reduce the food.
Some of this in my son's case is lack of teeth. He only has 2 of his 4 molars, meaning he can't really use the molars properly. I suspect the other part is that he is used to doing what he does since he's never needed to do anything else and it works well enough - he learned from a very very young age how to avoid chocking, after all, so it doesn't phase him at all. He just gives me a present (sigh).
What we do about it is two fold. First, just be aware of it and cut food up properly for him. Big enough pieces that he wants to at least do something other than directly swallow, but small enough that it's not a real choking hazard.
Second, we push him periodically with choking-safe foods that need real chewing. Green beans, for example, when cooked properly are great for this - they're not that hard to chew, and they are long and thin so unlikely to actually choke on, but they are big enough that you want to chew them. We've found that he is starting to chew his food more, and we're able to give him bigger pieces more often.
You also can try playing chewing games with her - ie, put some food in your mouth and make silly exaggerated chewing motions. She should see you doing that and (sometimes) try to imitate you - give her some bread even in this case, something very easy to chew but allowing her to mimic the motions. Children at this age love mimicry and find it irresistible.