Take a ladle of water from the spaghetti pot and pour it into the pan. Reduce the heat and leave. If the water evaporates, add a little more.
It is important to take water from under the spaghetti. The fact is that during cooking, the pasta gives up some of its starch. Thus, the water acquires adhesive properties. Therefore, in all sauces and other things, it is better to take water from under the cooked spaghetti.