Some irregular verbs

Category 1: irregular ending with no vowel change, V2=V3

group 1: d ending changes to t, ent changes to end 

bend     bent    
build     built     
lend     lent     
send     sent    
spend     spent    

group 2: the verbs have and make 

have       had     had
make     made     made

group 3: regular (V2, V3) in US English, irregular in UK English 

learn    learnt    
burn     burnt  
dwell      dwelt  
smell     smelt    
spell      spelt  
spill     spilt    
spoil     spoilt  

group 4: ay ending changes to aid

lay   laid    
pay    paid  
say    *said (/sed/)

Category 2: Vowel change, irregular ending, V2=V3

group 5: vowel change , d or t ending 

creep     crept  
feed     fed  
feel     felt    
keep     kept
kneel     knelt
meet    met  
sleep    slept  
sweep    swept  
weep    wept  
*leave    left