The verbs that carry the grammar behave differently to those that bring the meaning
There are several different types of grammar verbs: auxiliaries, modals, modal auxiliaries and one primary verb–to be. Each type behaves differently and you need to know this behavior to use if correctly.
Almost all sentences in English contain a grammar verb. They are the motor that drives English construction. Above all 15 centuries of history have made them more and more central.
This is an important point to understand for Italians. Italian main-verbs are much more grammatically autonomous, and grammar verbs and main verbs share the many of the same properties. A lot of mistakes can be eliminated by an understanding of the difference in behavior of the grammar verbs in the two languages.