Lessons abound. I was standing by my car the other day, waiting for a friend to finish shopping. Someone parks in the space next to mine, walks toward the store, and then turns around abruptly to retrieve his pandemic mask. As he passes me again, he points to the mask and says, we’re going to realize one day we should have did this. A consultant being interviewed on the news laments, he could have ran a good campaign, and an avid runner after a disappointing race confesses, I should have ate better. Each of these statements evinces the same grammatical lapse, and a brief explanation here might help prevent a relapse.
There are, you may remember, six tenses in English, four of which are compound in form, meaning that more than one verb is needed to compose the tense. In, for example, the statement I have traveled to Europe quite often, the verb comprises the two words have traveled, not merely the word traveled, and when such is the case, the compound verb makes up what is called a verb phrase. This terminology can be helpful in sorting out the structure of more involved clauses, particularly when they include other verbs not strictly part of the verb phrase. The statement I have wanted to travel to Europe for years includes three verbs (have, wanted, and travel), but only two, have and wanted, comprise the verb phrase; the infinitive phrase to want stands outside that verb phrase and serves as its object.
The last verb in a verb phrase is called the principal verb, and any other verbs in the phrase are called auxiliary; auxiliary verbs must precede the principal verb. Some auxiliaries help to build the tense; others serve to construct what is called the voice or mood of the verb. Three of the four compound tenses (called the perfect tenses) use some form of the auxiliary have in their construction, and here is where we come to the problem at hand: the principal verb that follows the auxiliary have must always be in its participial form. Almost all verbs have three parts (drink, drank, drunk; go, went gone), and the last of these three is called the past participle. This is the form that must always follow the auxiliary have in the construction of a perfect tense: I have drunk, I could have drunk; she has gone, she should have gone.
So, if now we analyze the statement we should have did this, the verb phrase comprises two auxiliaries, should and have, but the form of the principal verb following have is not correct. The three parts of the verb do are do, did, done, and the speaker chose the second part, did, instead of the third part, done; correctly, then, the grammar should be: we should have done this. Likewise with the other two examples. The three parts of the verb run are run, ran, run, and so: he could have run a good campaign. And the three parts of the verb eat are eat, ate, eaten, so standard grammar dictates: I should have eaten better. If you’re in doubt about what the parts of a particular verb are, a quick look at the dictionary will give you the information you need.
As we can see, grammar can accelerate quickly, even in statements as common and familiar as the three in question here. As ever, though, if your analysis proceeds methodically and patiently, the system of grammar reveals itself, and its requirements serve (as in all the arts) as limits or boundaries within which new thoughts and clear ideas can take shape. The rules of grammar are better thought of as the rules of a game: arbitrary in the grand scheme of things, but necessary for the communication of creativity and meaning.