Perhaps you think that "irregular" means something out of the ordinary, and in common parlance it does, but if you look up irregular in the dictionary, you'll see that the first definition is a little different. "Irregular" describes something that does not adhere to established rules.
Irregular verbs live by their own set of rules; they are the mavericks of the English language. Most of the verbs in the English language are irregular verbs.
A regular verb, like "to walk" looks like this:
You've known the tenses since you were a child. In fact, when English-speaking children or English learners come across unfamiliar irregular verbs, they instinctively employ the regular form.
For example, when a child hears "speak," without any prompting, he will say "speaked" when speaking about the past. When people hear it, they might think it's cute, but it should teach everyone interested in such things a valuable lesson about irregular verbs.
Any verb that does not break down using the rules of tenses like "to walk" is an irregular verb. That means that the conjugation is not based on the rules of tenses; but rather on the specific conjugation which is used for the word.
This means that verbal conjugation is a game of memorization. You need to memorize the specific conjugation for each irregular verb.
Since there are no specific rules regarding conjugating the tenses of irregular verbs, the only way to really learn them is to keep your ears open. Every time you learn a new verb, make sure you know all the conjugations of that verb so that when you use it you don't misspeak.
Here are a couple verbs that are commonly misspoken to get you started on your irregular adventure:
The English language works, more or less, without paying too much attention to whether the speaker is using the verb correctly or not. When an English learner says, "yesterday I speaked with the President," it makes as much sense as if he'd said "spoke." That flexibility is why English remains the lingua franca; one does not have to speak it properly to be understood.
Since people can understand English even when it is spoken incorrectly, using an irregular verb correctly isn't critical to be understood. Other languages have conjugation rules that are all but required just to get a simple point across. In English following the rules are not critical to be understood. That's why you can use English to buy tea in Thailand or sphinxes in Cairo.
If you want to sound intelligent or at least like a native speaker, you should mind your irregular verbs. They aren't hard to remember, and they really do make a difference when you're speaking with anyone whose opinion you care about. Having a firm grasp on one's irregular verbs displays a command of the English language.
Teachers, bosses, and English majors cringe when someone says "I should have drank" or "I should have went." Such sentences are understandable but just plain wrong. Correctly, they should be said "I should have drunk" or "I should have gone." Sometimes conjugations are nebulous or dependent on how English is spoken in that part of the world; but, most words have three forms, and it's good to get them right.
The most common English verbs, like "go" or "have" or "be," are all irregular and take a bit of practice to master. When you have your irregular verbs down pat, you won't be able to help noticing how many people have them all wrong. Getting them right shows a pride and responsibility that should be more common in English speakers.
English is the most versatile and expressive language on the planet, and its native speakers should put forth the effort to be a good example for the rest of the world, most of whom are spending a great deal of money and time learning it.
Most importantly, if you know your irregular verbs, you'll sound smart.