In English we have three auxiliary verbs:
“Do”, “Be” and “Have”.
But why? And what do they mean?
In grammar we often talk about “tenses” to talk about time. In fact, a tense is a little more complicated than that, and that’s when the auxiliaries become important.
A grammatical tense is a combination of time and “aspect”. Aspect is the “status” of an action (verb). The auxiliary verbs tell us the status of an action (verb) at a particular point in time.
“Do” is used with the “Simple” aspect. That is to say the most basic or general information about an action.
“Be” is used for the “Continuous” aspect. That is to say that an action is incomplete at a specific point in time.
“Have” is used for the “Perfect” aspect. That is to say an action is complete at a specific point in time. It also indicates the action had a connection to something earlier.
#1. “I (do)* build boats”.
This is the simple aspect and tells us a generality. In fact, we don’t know if I did yesterday or will tomorrow, technically speaking.
#2. “I am building a boat”.
This is the continuous aspect and tells us that the boat is not finished, but is expected to be finished at some point soon.
#3. “I have built a boat”.
This is the perfect aspect and tells us that work was started earlier and completed; the boat’s construction is completed.
There’s a wonderful simplicity to grammatical aspect, until we arrive at the fourth: the “Perfect Continuous”.
Ok, we seem to have a paradox. An incomplete/complete action. Also, we don’t have a fourth auxiliary verb to deal with this. So what on Earth do we do?
In fact, the perfect continuous describes a “partially complete action” and so we use a combination of “have” and “be” to make “have been”.
#4. “I have been building a boat”.
This tells us that work has started on the boat and progress has been made, but the boat is still not finished.
The key insight here is that English only has these four ways of describing the status of an action at a particular moment in time.
In combination with the three times, past, present and future, we can create all tenses possible in English. The status is just moved to one of the three time periods.
* (We omit “do” in positive direct statements, we only use it to insist).