Most English verbs conjugate simply and regularly, but the most frequently used ones are exactly the ones that don't — and a separate category, phrasal verbs, adds a genuinely distinct layer of difficulty on top.
"Go/went," "be/am/is/was," and "have/had" are among English's most irregular verbs — and also among its most frequently used, meaning learners encounter irregularity constantly from the very first lessons rather than only in rare, advanced vocabulary.
English combines common verbs with small particles (up, on, in, out) to create phrasal verbs whose meaning often can't be guessed from the individual words — "give up" (quit), "look forward to" (anticipate), and "pick up" (learn, collect, or several other meanings depending on context) are not literal combinations of their parts. These are typically learned as whole units, not decoded piece by piece.
No — the vast majority conjugate regularly (add -ed for past tense), but the handful of highly irregular verbs (be, go, have) happen to be among the most frequently used, so irregularity shows up early and often.
A verb combined with a small particle (up, on, in, out) that together create a meaning not obviously guessable from the individual words, like "give up" (quit) or "look forward to" (anticipate).
As whole units — their combined meaning usually can't be reliably decoded from the verb and particle separately, so they're typically memorized as fixed expressions.
They're expected to be, given how frequently they appear in everyday English conversation.
See also: English Grammar · English Vocabulary · Learn English