One of the keys to eating a Hass avocado is knowing when it is ripe and ready to eat. Cutting one open before it is ready can be disastrous because the flesh will be too hard and inedible, and once it is cut open, it may never ripen properly. You have to look for certain signs that tell you when it is ready to eat.
The flesh of a ripe avocado is soft and a bit mushy to the touch. Feel around the circumference of the fruit with your fingertips, applying gentle pressure as you go. It should feel soft all the way around without hard spots that do not give way to gentle pressure. Sometimes one area of the avocado will ripen before another, and you need to make sure that the entire fruit has softened before you attempt to eat it.
You may use coloration as a guide also, but it is not as good of an indicator as touch. As it ripens, the skin will turn from green to brown and then finally to black. There is often a mixture of these colors as you look around the circumference of the avocado. The blacker the color, the more ripe the flesh should be on the inside. Take this with a grain of salt, however, because sometimes the color changes on the fruit but the edible flesh never properly ripens.
It can be tempting to cut open an avocado when the flesh is too hard, especially when you’ve been waiting patiently to enjoy its exquisite taste. Doing so can ruin the avocado, so I don’t recommend it. I do recommend putting the avocado in a brown paper bag in order to speed up the ripening process. It works even better if you put other types of fruit in the bag, especially bananas. The ethylene emitted by ripening fruit helps to speed up the process.