Learning a foreign grammar mirrors our experience growing up. First, we are told that there are rules of correct behavior. Then, we discover that some people don’t have to follow them. Similarly, students learn verb conjugation tables – er, ir, re in French, the famous binyanim in Hebrew, to name a few, and then continually run into verbs that do not exactly follow those rules. Like the confused child, the learner gets the impression that the rules were a big lie.
The first logical question concerns the reason for the very existence of these exceptions. The obvious but true answer is that because they were always there. Native speakers, even today, learn most of their grammar through listening and imitating, not formal study. Most first language native speakers cannot explain their choice of verb tense logically, knowing what is correct by what sounds “good.” In other words, it is correct if we are used to it, not if it follows some academic rule. On the other hand, if people rarely use and hear a given verb, rendering them unsure of the correct form, they will go with the general rule. For example, the past simple in English is made by adding ed to a word. Therefore, when unsure, about a form, the speakers generally follows the rule or looks for a similarly sounding known verb. So, while it is clear to Americans and Brits that the past of sit is sat, most speakers would say that the past of shit is shitted, although the phonetic similarity would lead some to say shat.
In practice, this means that the verbs people use regularly have a nasty tendency to remain irregular. Speakers who say “I goed” are corrected even though the conjugation clearly follows the formal rule. On the other hand, confusing exceptions such as lie (lay, laid) and lay (laid, lain) are often mistaken (except by English teachers, of course) without causing undue comment. Therefore, the key to irregularity is use.
An example of an exception proviing a rule is Hebrew. Hebrew was basically static for some two centuries. This period allowed scholars to devise rules to explain almost all exceptions. This structure, called binyanim effectively organizes all Hebrew verbs. Most languages, especially English, have never had such a period to allow the scholars to catch up with actual use.
A deeper explanation of why English grammar in particular has so many exceptions will be the subject of another post.