Go is an irregular verb, and our answers wend their way back to Middle English and beyond. Fannish examples will come from Sapphire and Steel, who are accustomed to dealing with irregularities.
Regular verbs in English follow the pattern of adding es/s and ed/d to the basic form to conjugate different tenses.
I, Mab, dress in whatever I can find in the mornings.
I, Mab, can't dance because I have two left feet.
Sapphire dresses in blue.
Sapphire dances gracefully and will probably flirt with you if you compliment her skill.
Steel dressed in grey.
Steel danced divinely, but he didn't appreciate compliments over such a trivial skill.
Irregular verbs don’t follow those tidy rules and can confuse any new learner of a language. They also confuse lolcats. “I made you a cookie but I eated it” is a pretty well-known example of how cats don’t deal so well with irregular verbs. Irregular verbs in English are words that we use all the time, and so the proper usages are generally ingrained into native speakers.
Things Steel and Sapphire might say:
We go back in time, but only in terrible emergencies.
We went back in time to save the world.
Our agents have gone into the past because humans can’t learn to leave well enough alone.
Going back in time is dangerous and not undertaken lightly.
Things they most certainly would not say:
We goed to Earth because our transdimensional authority said we had to.
Lead and Silver had went into many situations that humans would find strange and eerie.
We wented into dangerous situations. (Although they might declare that they wended their way through history.)
If you've learned English as a second language, or you're finding that your ingrained native speaker knowledge isn't as deep as you'd like, here's a site with a full conjugation of to go.
Go, along with its fellow irregular verb be, has what is known as a suppletive past tense. That means that go and went are derived from two entirely different root words.
Go derives from the Middle English gon and goon, and the Old English gan, with further roots back to ancient languages, and related words sprinkled through Germanic and Scandinavian languages. When Old English wanted a past tense of go it used the past tense of another separate word, namely eode.
Went derives from the past tense of the Middle English word wenden, which we still know as wend, although it’s a word that nowadays is used poetically or satirically.
Somewhere in time – around the fifteenth century – went became the preferred past tense of go in southern English usage. Wend developed a new past tense, wended, and eode and its forms rode off into a dialectical sunset.
The usage of southern England became the standard usage, although differing dialects remained, and so go and went became the regular (or should that be ‘irregular’?) words that we use today.
Green-Grrl's lolcat post
Irregular Verbs at Really Learn English.com
Wikipedia Entry on 'Go'